RoP:I makes it clear that a demon's Infernal Might can be eroded, for example by Demon's Eternal Oblivion and that if completely gone then the demon's soul goes to Hell. What happens though it suffers partial erosion? I cannot see anywhere where it states whether Infernal Might can be regained. [I can find how might points in the might pool are regained up to the level of Infernal Might - p35/36]. Is this covered somewhere?
It occurs to me that if an Infernal Might cannot be regenerated then over time there will be a tendency for the population of demons to tend towards low levels of might. New demons would surely tend to be of lower hierarchy score types and older ones will occasionally meet something that causes them to lose Infernal Might. Perhaps there needs to be some way of demons gaining experience as a counter to the inevitable slow loss they experience.
This has been asked before - I think the answer is a resounding "no" - as in, "no, it is not defined anywhere."
Magical Might, and magical creatures, from any Realm, should not be rigidly defined or easily predictable. Maybe it regenerates quickly, overnight, and maybe slowly, over centuries. Maybe a MM of 0 destroys the creature, maybe it cripples them, maybe it just banishes them, maybe it just really pisses them off to no end, depending.
A SG should not be purely random and arbitrary, but nor do, for instance, two different types of demons have to work the same.
I did feel it had to have been asked before and I did have a quickish look in the archive. I think you're right, that it should vary. I also believe that any regeneration should take a long time, relative to the pace of events in the mundane world, by which I mean months or years.
Perhaps demons that have come from Hell, rather than have become demonic on Earth or in another realm, cannot regenerate. Another option would be that the demon has to return to Hell to regenerate their Infernal Might score.
The real answer is: whatever best fits your story IMS they regenerate might, but I have never really stopped to think much about it. I like the idea of them auto-banishing themselves if they are in danger of being destroyed by DEO. Then they need to be summoned to the world again from hell: since they were not destroyed they would still be around. And this is why you have so many books detailing how to summon a demon: they made sure they can return!!
That is quite a high fantasy approach, but it can work there
From page 35 of ROPti: might pool .. refreshes at a constant rate .. all might restored in a day. In an Infernal aura or regione .. recover spent might points more quickly ..
Number of hours to restore all might: 24-(2 x aura)
I personally don't see any difference between might lost to cost of use of the demons powers and might lost due to being hit by a DEO. If a demon is banished to hell (Infernal regio lvl 10) due to DEO then it recovers all it's might in 4 hours, but won't necessarily recover anything wounds and definately won't be able to turn up to fight until it is summoned again.
Actually, if you DEO to "death" a demon, you actually kill him: he stops being a minion of hell to become a soul tortured in hell. You do not erode its might pool, but its actual might score. Hence the difference.
Against a Might 10 demon, 2 DEO of level 5 with total casting roll of 15 each is just as effective as 1 DEO of level 10 with casting roll of 20. If your arts are high, you might do the high level one (esp since you can master it to multicast). If your arts are lower, the level 5 might be better.
Against a mighty 20, you are talking DEO lv 5 only needs a 20 to pentrate while 25 for DEO 10 and whopping 40 for DEO 20.
I recently made a Flambeau Demon hunter and realized that DEO 10 multicast was the way to go since she could count on a 20 penetration regularly. On the other hand, against a demon with might 30 (unless she does various things to boost her penetration), she is unlikely to penetrate while if she had a DEO 5, she would have a chance.
However to multicast, you do need to spend 15 xp on mastering the spell and that same 5 might put your arts where the higher level is better.
Only if your Art score is 0 or 1, in which case 5xps will raise you two points (from 0 to 2 or from 1 to 3). Otherwise it is break-even, because the Mastery score of 1 adds +1 to your casting total. I presume you meant 5 and not 15. A Mastery score of 1 and multi-casting allows one copy of the spell plus the original; two spells total. Now, when it comes to mastery scores of 3 or 4, then yes, there is definately a diminishing return. But if you take Penetration as a mastery ability, then you have a signifigant boost because then the mastery score adds twice (once to casting, and a second time when calculating penetration).
You should see a difference. We're talking about loss of might with DEO rather than loss of might pool. A might 20 demon that uses 15 of its points still has a 20 might and a 20 magic resistance. If that same demon gets DEO'd down to might 5, it is now a might 5 demon not a might 20.
You know, I am thinking there is a way to have bboth options. The standard Might stripper, taking away Might equal to it's (effective) level as long as it Penetrates; I am thinking that should come off of the Might Pool. Then it would take a separate and more powerful spell to strip points off of Might Score. Use the old rule maybe? Reduce score by a number equal to the amount by which the (effective) level exceeds the current Might Score (or equal to the level if a ritual). That way you can have the best of both worlds; permenant destruction of demons or temporarilly weakening them. In fact, this would put might stripping spells back in line with Wards, so that it is no longer easier to obliterate a demon than it is to ward it. This idea would make it easiest to weaken a demon, followed by Warding, and Score Reduction/Destruction would be the hardest of the three.
Yes, and if you have someone in your group with 5 points in Spell mastery DEO and a reasonable Perdo-Vim, there's no fun in demon's anymore .......unless they are incognito.... >:-)
Maybe it needs an infernal aura of at least might/5 in order to replenish its score, with a number of points per season equal to the difference between the two? Hell allowing in any way at least 1 point per season?
IMS they regenerate might, but I have never really stopped to think much about it. I like the idea of them auto-banishing themselves if they are in danger of being destroyed by DEO. Then they need to be summoned to the world again from hell: since they were not destroyed they would still be around. And this is why you have so many books detailing how to summon a demon: they made sure they can return!! 
