A few dark & evil cults:
Tremere Necromancers
This is a secret mystery cult that controls House Tremere's higher echelons. Its outer mystery is the Leadworker virtue, and it adds in suitable necromantic virtues like Second Sight, Spirit Familiar, Canaaite Necromancy (integrated from Ancient Magic) and the Living Ghost. The cult is secretly led by liches - living ghosts possessing a human, whose spirit returns to their talisman if their body is slain. The liches control House Tremere as well as (often indirectly) other magi and resources.
The biggest secret of this cult, however, is that it is actually waging a shadow war in Tartarus. Much of the raw vis and magical resources of the House are spent on this war; and as the liches are losing, they reach out to the PCs. What they are fighting is Tremere's Army - the army of ghosts, bound titans, and other fell beasts that Tremere the Founder - or rather, his ghost - marshaled in Tartarus after his death. He apparently didn't despair of the idea of ruling the world; he just decided to rule the underworld first, and now he's just about ready to start on the upside again...
Astrologers
Renowned throughout Mythic Europe for their open schools on Artes Liberales (each master teaches once a year, free of charge, barring none from his lectures), these magi are seen as scholars employing Divination through Astrology to provide uncanny insights into the future. Many a king and wizard consults an Astrologer regularly.
The cult starts by teaching Affinity in Artes Liberales, then moves to Celestial Magic and a suitable Magical Focus (Sign); with Death Prophecy as an Ordeal. Only through long service can one then reach the second circle, which indeed teaches a variant of Divination, but also grants Visions.
The big secret of the cult, however, is that it is actually Infernal. The Visions are demonic, and allow demons to subtly corrupt the practitioner. Cultists are taught a cyclical-time heresy similar to that of House Criamon, but with en emphasis on the suffering in life and the impossibility of breaking the cycle. Cultists believe going to Hell in their afterlife is their Fate, and deny Free Will. The inner-most circle is taught Cthonic Magic and various Infernal tricks I never got around to describing. These should definitely include the ability to use Intellego to Hex, and do Premonitions.
They are basically bad-ass because they know everything that is about to happen in advance, and use various Infernal tricks and subterfuge to turn that to their advantage. They posed as allies, and were supposed to have been manipulating events to serve their Infernal aims in the big reveal - including the fall of the covenant and tribunal to the Mongol hordes.
Dreamers
A cult that teachers dream-magic. They have a bene-gesserit thing where a master (physically) steps into the dreams of his disciple, continuing to live in his dreams. They thus harbor immortal, ancient, magi inside them, which they can let possess them at their will. They are also heavily into the Shared Unconscious, seemingly engaged in a strange war that occurs in dream-time between the Jungian Archetypes across the dreamworld of all of humanity.
For the most part, they are simply crazily weird magi that can jump out of people's heads, turn into any archmagus type in an instant, speak in dream-logic ways (and achieve things this way!), and generally serve as as the wildcards and hail-marys to save the PCs bacon and be the "Yoda" of the saga.
The Masters Beyond Death
This dark Merinita cult worships the Babylonian god of the underworld, Nergal. It’s members practice a form of corporeal necromancy, raising undead minions to do their bidding. It is unclear how recent the cult is, but in its modern form it primarily relies on the Mystery of Creaturecrafting.
Only practitioners with Faerie Magic are accepted into the cult. It is possible, however, to join House Merinita and hence be initiated into this Outer Mystery and then join the cult.
First Initiation: Cult Lore 0
Mystery: The supplicant gains Minor Potent Magic creating Undead.
Ordeal: The supplicant gains the Oath Flaw, representing his oath of allegiane to Nergal.
Second Initiation: Cult Lore 1
Mystery: The supplicant gain the Creaturecrafting Mystery.
Ordeal: The supplicant gains the Sensitivity to the Divine Flaw.
Third Initiation: Cult Lore 3
Mystery: The supplicant gains Major Potent Magic in Undead.
Ordeal: Loses Minor Potent Magic.
Names of Power 4
Consumate Talisman; Deleterious Circumstances 5
Fourth Initiation: Cult Lore 6
Living Corpse
Crafter of Undead: Minor Mystery
Through this Mystery, the disciples of Nergal create powerful Undead. Undead may be created as Magical creatures, with total Might equal to five times the raw vis invested in the casting. (This is usually twice the spell’s level, but can be less if means are taken to reduce the Ritual’s cost, such as Mercurian Magic.)
Creating a magical beast is Creo Animal Base 50, with Vim and possibly other requisites. This Mystery opens two new base guidelines, Rego Animal and Rego Corpus Base 50, for creating corporeal undead. As with CrAn, a Vim requisite is always needed, and further requisites may be required. The spell must be a Ritual, and it can only be cast on an appropriate target. The Masters Beyond Death almost always use the Until duration, with an appropriate condition that will break the spell (see below for examples).
The created undead are created as Magical characters, with appropriate Might and powers. If using Realms of Power: Magic, they should be Spring characters. If not follow these guidelines: their most powerful Power should not exceed in Level + Penetration the Ritual spell’s level, they should have no more than a few Powers.
Who can be raised? The targets of the cult’s spells are often corpses. The spells fail to work if these have been given proper Christian burial, or the spirits are otherwise in the Divine realm. Spirits in other Realms are usually less vulnerable, but it’s possible that particular ones will be, depending on their fate in the afterlife.
Raising the Undead Horde
ReCo(Vi) Level 95 (B50, +1 Touch, +4 Boundary, +4 Until) Ritual
This feared ritual raises an army of lesser undead - skeletons following the orders of the caster. It can be cast on any collection of mostly-whole skeletons or corpses, such as those from a graveyard or field of battle. All the dead in the area rise up, climbing out of graves if necessary, and present themselves in service to the caster. The spell only animates their bones, and doesn’t protect their flesh from rotting further, so that in time they will turn to skeletons even if at first they may appear as zombies.
The skeletons are loyal and tireless, but not clever or particularly skilled and completely lack any ability to innovate or deviate from their orders. While disciplined, they lack the ability to function as a Group in combat. While terrifying, their combat effectiveness is thus limited. Their one advantage is that they can be created in huge quantities, from the bodies of slain enemy soldiers: because of their minimal Might, these skeletons can be created in huge quantities.
The magic animating the skeletons is undone if they are ever commanded by someone with rightful authority over them, such as their mundane Lord or their military commander. It will also falter if the skeleton is physically destroyed.
Skeleton of the Undead Horde
Magic Might 1
Intelligence -3 (literal thinking), Communication -3 (silent), Presence +1, Perception -3 (dim senses), Strength +2, Stamina +3, Dexterity 0, Quickness -3
Abilities: Speak Native Language 3 (his native tongue)
Virtues and Flaws: Small Frame (skeletal)
Initiative:
Powers: None.
Equipment: Leather hauberk, axe.
Conjuring the Shadow of the Icy Grave
Re(Pe)Co(Ig,Vi) 75 (Base 50, +1 Touch, +4 Until) Ritual
This dark ritual creates a powerful, incorporeal assassin in the caster’s service. It can only be cast on a body buried in deep, cold catacombs.
Undead Shadow
Magic Might 75
Powers:
Incorporeal PeCo 60 (B40, +2 Sun): The undead corpse becomes immaterial. It appears as a black shadow, and can slither over the ground but has no solidity or substance to it.
Icy Grip PeIg 10 (B5, +1 Touch), Penetration 65: The shadow’s touch does +5 cold damage. It is cold enough to freeze water.
The Fury of the Lost of Pompeii
Re(Cr)Co(Ig) 75 (Base 50, +1 Touch, +4 Until) Ritual
Burned Men
Magic Might 75
Powers:
Touch of Abysmal Flame CrIg 30 (B25, +1 Touch), Penetration 35: Touch does +30 damage.
Arc of Abysmal Flame 45 (B25, +2 Voice, +2 Group), Penetration 30
The Corpse Golem
Re(Mu)Co 85 (B50, +1 Touch, +2 Group, +4 Until) Ritual
Targets 10 related corpses, all must have died in the same battle, for example.
Magic Might 85
Powers:
Swallow the Dead MuCo
and a related spell:
Forging of the Hauweta
Rego Mentem (Corpus, Ignem), Ritual
R: Voice, D: Permanent (non-Hermetic), T: Special
This spell is cast on two persons that are burned together on the stake as part of the casting. The two must be connected mystically in some way - Dracul, the inventor of this spell, used it on women pregnant by a man's seed, casting this spell as a punishment and trial-by-ordeal for infidelity.
The spell binds the spirits as they leave the body, forcing them to remain in the burning bodies and obey the caster's commands. It also binds the bodies together, creating an ever-burning mesh of bodies and souls.
The resulting undead, called Hauweta, always refers to himself as "we", and speaks earily in two voices. It is in eternal agony, forever burning in unnatural fires, but is forced to obey the caster's commands even through its suffering. Hauweta are immensly strong, but fragile (Dracul wraps them in metal armor). In addition to their martial capability, magic resistance, and utter loyalty, a Hauweta can make others share its endless pain (R: Eye, D: Diameter, T: Individual; Me; cause immense, debilitating pain).