This sounds super fun.
That is all!
I like this a lot. I see some of the parens opposing this enterprise. At first the reasons are unclear, but the character in time unveil them. It could be because the old magi fear that by getting in contact with the evil, their filii can be tainted and tempted by it. Or it could be because to defeat the evil lord the first time they had to break the Code or do some other sort of crime, and they don't want this story to resurface. Or it could be because one of them is allied with the lord or took his power for himself, or it has always been him.
Yep! Totally, exactly. I was thinking similar line of thought. I don't entirely know the answer of which way I would go, but they are all of potential and interest. I especially like the idea of one of the parens who you always thought was great turns out to not be so great! I also like that there are plots within plots and stuff, that people who you think support you in your activity turn out to not actually support you. And heck, maybe even people who you think are enemies turn out to actually be trying to help you. The idea of the circle of parens who defeated the dark wizard before doing it in a totally not fully acceptable by the Hermetic Code way, sounds interesting.
So yes, very much so, these are some really great ideas. 
Revenge is a good motivation for a dark lord, and it is one I'm using.
The "finding/funding allies" part of any dark lord rising is currently what drives my saga IRL. Creating alliances is important, because having pawns is important, but not enough. And to find allies when you are someone with a dark goal, it is not simple, your allies are probable dark creatures themselves, which you should treat carefully and cautiously.
The enemy must, IMO, be distant and "abstract" at first in the saga, to give the PCs a sense of mystery. A dark lord who is known from the start is a threat the PC can give away to their hierarchy/house/friends...
Something i have found interesting is to use the name repetively, but never actually confront PCs with the enemy, but with minions, or make references, and indirect ones to this growing power.
I do, personaly, not find interesting if the dark lord has already his plan working correctly when the PC start knowing his existence. In that case it's more a "kicking dark lord" saga. Using prophecies/visions to show what will be in the future is always a good trick.
Having a set "end" to the dark lord reign/rise is interesting because, even if the PCs know when or even how it ends, they do not know what the destruction the dark lord will cause until that point. And sometimes history is more about what has been done during the journey than what happens at the destination.
If a dark lord starts by making "normal" acts, then it is not a problem if he is known - obviously -, but imo, it's not a dark lord then. A dark lord is a dark lord if his goal is stated to the PC at least. Until that point, he is just "someone" (powerful, maybe) with ambition, and magi are full of those, allies or enemies. YSMV of course.
Putting enemies to the dark lord which are not only "good" characters like the PC may be is also an interesting options because it proves that the rising dark lord is a danger for everyone, even "bad" people. In that sense, he is truly a dark lord which wants to reign over everyone and everything. There will be only one lord: himself. Other dark lord -to be - would then ally with people they feel confident in their desire to struggle... like... say... the PC. And then it's a similar story as the known "how far will the Jedi ally with the Sith against a Celestial?" (for those who have read Fate of the Jedi saga in starwars extended universe)
If the magi you get are a pious bunch, consider a saint or an angel for their patron spirit. While the Divine isn't going to drop a hammer on the Dark Lord directly (Satan has the right to test mankind), if these magi turn to God for aid, then He will answer.
I agree with you for the most part. I think that at the begining of the saga having it be known that he is not dead but not really knowing what state he is in or even where he is would do a lot to give it plot importance but not entire focus. (In some ways its like how Harry knew Voldemort wasn't dead, but it doesn't mean he stopped going to school just because of it.) While tracing the dark wizard's past and activity the group would meet a bunch of people who met him, knew him, or worked for him in various ways - so mostly minions, agents, and innocents effected by his dark actions.
At some point this would change and said dark lord would rise again, probably in a ritaul that the whole 'world' can feel or something, to showcase the saga is moving into a new era of focus.
I am not sure what you mean by "I do, personaly, not find interesting if the dark lord has already his plan working correctly when the PC start knowing his existence." I mean he was not supposed to be defeated, it wasn't to plan, and his state is a little less than death and again its not to plan, so its not like its what he wanted. The players know he is not fully dead but that is all they know, not why he is not dead, or how he is not dead, or where he is in his not dead state. I'm curious on how this is not interesting, unless its just not, than well that's cool, its not for everyone.
When he returns and is active again its going to be a bad thing for the world, probably only made survivable because over the last decade or so the player magi have attempted to figure out his defenses and stuff. His return is going to probably pull in some sort of apocolyptic setting - maybe the aether or winter or the gods of magic or something else, maybe he brings about a plague which he is immune to so that he can take over the remains of the world, I don't know, but basically when he becomes active again its going to be BAD!
Your point about him not being a dark lord but just a bad guy is a good point, and its why in fact that when I refer to his pre-destruction antics I call him the dark wizard, but its only after he comes back and decdies probably that there is no point in trying to keep the world intact, that he becomes a dark lord, fully and entirely.
I like the idea that as the players progress they will have to connect with and even ally with people who they might not have done in earlier times, that sort of morality decision making could be interesting.
Very cool ideas, thanks for the comments. They are quite thought provoking!
I guess for me its hard to want there to be a Divine force and stuff, I prefer keeping it Magical (and if I have to Faerie, but even that is pushed to last choice). Its the same reason that I don't automatically invision all bad guys or whatnot to be Infernal, and I don't really like the idea of the Dominion coming in and saving the day and stuff.
That said, in the Theban Tribunal there are a number of covenants that have divine patrons that are kind of interesting and stuff, including the one with I think its Demetrius in Macedonia, that place kind of gave me ideas. hehe
Heru, thanks for replying.
First, understand that it is my opinion, so I may not really have "arguments" and have maybe feelings about this.
Now, for what you didn't catch:
I think that, if the dark lord is someone who is "known" beforehand, at large. Say a "Voldemort" type of enemy, when the PCs will find/discover (or, PCs being PCs, only "guess) that he is still on board and doing things, their first move will be: "just give the message to those magi above us. They have dealt once, they can deal twice." It is the type of issue I run into when Diedne or "marched" magi are encountered. If those are not directly opposing the PCs, PCs hardly try to stop them and give information to their superiors, even if noone in the covenant, in the Order at large (house, primi, praeco, quaesitors... potential persons are numerous).
At least MY players are the kind to think that only fantasy books let "noobish" magi deal with big enemies. They only do so when disclosure would mean personal failure for themselves (because of secret, reputation, etc.).
If the "enemy" is opposing them - they are just "no one" for him if he is a dark lord - then his scale of action goes from "dark lord threatening the order" to "person threatening the PCs" by design.
But, I face this choice :
- either he IS a dark lord and if the PCs are a threat at the first meeting while doing his grand plan, then he has no chance of success for his grand plan because the order has archmagi, and Houses. (Moreover, in that sense, the PCs will just not consider him to be a dark lord.) But if they are not a threat, then why would he let them live if they oppose him? Of course we can invent different reasons, but at one point, one PC will ask "but why are we still alive ? our saga focus is him, each time we leave the covenant it is to oppose him, directly or not, he has met us, and we are alive... huh?" and your reasons will sound a gaming point of view "PCs must not be dealt with too easily" which may destroy the credibility of the threat.
- either he IS a dark lord to be, not an active one, and he is building his (rematch) plan. At that point, his plan is not perfect, and better be careful. (Think Voldemort years 1-4.) He avoids confrontation, he avoids enemity. Secrecy and mystery are his tools. Building alliances with other enemies (to betray them eventually). PCs are just random people, and he don't antagonize them. Maybe he tries a discreet recruitment, or just a "good to have seen you , bye bye" kind of meeting. But his minions may, them, face and oppose the magi. And by those opposition, they understand what happens at large.
So using the second option, and considering the "minions" aspect of any dark lord (Voldemort & the death eaters [not sure of the name in English since I read the translation in French first and those kind of words are transformed to keep the wordplay], Palpatine and his hands and officers, Sauron and the nine, Saroumane &co, the Dark One and the 13 Fallen, etc.], I prefer to consider that while the Dark Lord (tm) is building the plan, he lets minions handle "day to day" operations.
And those minions are not part of a working plan they could betray if failing to (whatever they want to do to PC but PC capture them, for exemple), but have only information.
The PC can get report of the "I heard far that this is happening", but it is only later, when they start being "powerful" and thus "have less and less superiors at whom they could pass the information" the rising of the dark lord.
Besides, option 2 may lead to option 1 if the Dark lord succeeds (see later in this post about the "do you really do this? part of this apocalypse adventure.)
FInally, the plan is ready, and the PC have clues and will be forced to deal with in a "Voldermort year 7" denouement, unravelling the plan.
Using a previous "known" figure is always a good thing (coherence of universe for exemple), but this previous figure must not be too easily dealt as being "a pure evil to be destroyed". Grey is for that part of his life the best color because PC would want help, but helpers would say something like "na, it's not a threat, besides he is dead, it was a long time ago". For that matter, Diedne is perfect because most current houses (except Tremere? Guernicus?) have a more conciliatory approach than they had in the start of 11th century.
So in short, I think that unravelling the Grand Plan just at the time the Dark Lord will start doing it is interesting, but if one really wants to establish the Dark Lord as a dark lord and not just "another one of those foolish enemies who will do some deads then bye bye", you must have sacrifices of covenants/magi. Big destructions ring bells, thus, a slow progression is better. You can have a turning point where the rythm starts going crazy, and then the PC will have figured he is the mastermind, and it is time for "the reveal" with some big important covenants to blow and shatter your Order. Me? I would target a certain House who is a bit too much known to have contingencies for everything. Cut the so called head, and let the feast start. (I'm not being too descriptive because at least 1 player of mine reads this forum.)
Obviously: since most games do not want to face destruction of the order (meaning changing the setting too much), you can foresee this, and let the PC plant a solution for the future because they know the "solution" (chosen one etc.) must grow, learn. They know they are not meant to stop the dark lord, but to mentor the One who will. That is really a personal choice.
Obviously, using twilight to pass such information is fun. So you can have a "apo" kind of adventure while all magi are in twilight. And that is the time they can deal with a powerful and in place dark lord. Then after, return to the present, and the "oops, we must prevent that from happening" if they believe in it, or the "it was a strange twilight" kind of reaction for those who didn't.
I'm not sure I'm clear, sorry, i was much interupted and I have a hard time to organize my ideas, moreover in english.
IMHO, you have to go with the 2nd choice. If we stay on the Voldemort analogy, you need someone powerful enough both with magic and politically . The building years have to take out potential problems like Flamebeau and Tremere. Either politically ( get at least one of them on his/her side) or another Schism War with the Order divided between the two most martial Houses. Then the DL can rise up and "save" the Order or reform it.
On a tangentially related note, Voldemort himself gets his arse handed to him by any hermetic magus with a little time on their hands. Lack of parma, and an ego the size of saturn and it's rings means he'd expect to be able to take on and win against a hermetic mage, who, after killing him didn't work, would probably find their local mentem/vim specialist and staple him to the wall.
I will just pitch an idea that I used in a past campaign for one enemy I used.
The lady in question was half human, half-faerie (not just Faerie Blood), and her father was a powerful, cognizant faerie.
I ruled that since she was half faerie, whatever trace she would leave was only glamour fading away within a few hours - without even her realizing that. Thus, when the magi broke into her home to steal some arcane connections, there were sorely disappointed.
They could not believe that somebody would be so thorough with her hairs so they thought about a magus casting several times a day a spell to destroy any Arcane connection - quite a paranoid magus. Until they realise the true nature of their opponent.
They had the whole collection of location spells, TP and so on. I did not want to make the fight to easy with Arcane connections and I also wanted to give them a few hints regarding the nature of their opponent (they were completely unaware of her mystical origin). So I found a "fair trade" of depriving them of their favorite tools, yet giving them something in exchange that would be very useful.
It worked well and was well appreciated by my players. And obviously , it is not a trick that I could pull on them any time soon 
As has already been mentioned, that hoary old "return of House Diedne" could work really well in the role of "Dark Lord."
I'd be inclined to make it some close associate of Llewellyn the last Primus -- a lover, a child, an utterly-adoring apprentice -- Someone for whom it's personal: they want the entire Order of Hermes, every member, to die in screaming agony. Nemesis,
Give them Hermetic/Diedne magic, of course; plus some extras that they found/learned/won/bought/etc deep in whatever Realm/Regio where they've been stuck since the death of Llewellyn. Link them up with some Mystery Cult based around those "extra" bits -- to the MC, this "new" wizard seems like an ascended being, so great is their grasp of the Cult's Sacred Mysteries! Bonus points if it's a MC to which a PC or close ally belongs (at very low level, so they aren't privy to the Arrival of the Ascended One, and likely don't recognize THEIR suite of MC-powers in the amazing stuff done by Nemesis).
Bam: instant DarkLord, hates the Order, deep Hermetic roots, fanatical followers.
My big problem with this is that it seems to make the Divine a pushbutton "weapon," a force without any agency. In fact, the Divine is chock-full of agents who in fact DO NOT do evil acts; plus of course the Almighty -- the being of limitless will -- who has to WANT (or at least be WILLING) for Divine Power to work, before any "massive effects" can be generated.
The Divine/Infernal powers are characterized by having inherent moral stances, and each side having a Big Guy whose intentions more-or-less define what can/cannot happen with the power of that Realm. The Magic/Faerie powers are generally characterized by having no such overarching stance, or overseeing Being (I mean, sure -- lighting a fire in the palace of the Winter Queen will piss her off -- but not all of faerie!) .
I could believe the "Light Lords" idea as actually an INFERNAL plot... trying to make the magi think that God Is Against Them, to drive them into sin.
I could believe the "Light Lords" as straying-but-not-yet-Infernal holy orders: "We will fight fire with fire, and use these ancient Magical weapons against this infernal Order of Hermes" (using, as stated, Magical (not Divine) weapons, found by the Church and stored secretly for years).
I could believe the "Light Lords" were actually wielding Divine powers if there was in fact some new Infernal Corruption within the Order of Hermes, which some member(s) of the Order (the ones nominally in charge of remedying that problem) were too weak/afraid/etc to fight: the solution here wouldn't be to FIGHT the Light Lords, but to Clean Your Own House! Whereupon the Light Lords' special Holy Hand-Grenade of OoH-Spanking would cease to function, and they'd go home.