Basic Expert: Cults, Factions, and Other Planar Influences

Tomak the Undying, 75th level M-U

From Knowledge, Power
Of the knowledge to be had from other planar entities via the 5th level Magic-user spell Contact Other Plane or the 5th level Cleric spell Commune, there must be individuals and groups making use of that knowledge.
Primarily these will be Magic-users and Elves of a minimum 9th level or Clerics of a minimum of 7th level. Where Clerics enjoy three questions without any deception, error, or prospect of insanity, Magic-users and Elves do not.

The Ecologists
Their members are made up of various powerful sentient monsters, and Spellcasters. They use Artefacts of the Immortals as an energy source to power Dungeons. In particular they create permanently active spells at locations within their Dungeons where any creature dying is reincarnated randomly or animated as undead. Many Ecologists go out of their way to study the dungeon ecosystem, researching the weaknesses and successes of dungeons.

Agenda: The Ecologists are farming Adventurers for experience by using their living dungeons as a meat grinder that constantly grants them more experience as it chews up adventurers and creatures and uses them against the next wave of victims.
Membership: Any chaotic or neutral spell-caster who can learn of the secret of using dungeons to farm adventurers for Experience points via contact outer plane or commune, locate an Artefact of the Immortals as a power source, and research how to make spells permanant. They take an active interest in the ecology of Dungeons and are likely to hire adventurers to explore their own dungeon to test it, or explore some new dungeon they have learned of to pick up any useful information unique to its ecology.

Ex.-Tomak the Undying created a dungeon in the mountains that has an Altar of Raise Dead (crafted by a cleric friend) to lure adventurers seeking free* raise dead spells into a meat grinder that has reincarnation and animate dead spells at the bottom of death traps designed to lure the greedy to their doom and turn the newly deceased into monsters to restock the dungeon.

*meat grinders can be costly in resources.

The Brazier of Eternal Flame
Their members are Patriarchs drawn from other churches. Membership in this fire worshiping cult is kept secret simply because they dont desire the Scrutiny of mainstream faiths they consider Heretica. The Brazier of Eternal Flame communicate with one another by the use of a modified Brazier of Fire Elemental Conjuring. Having communed with other planar beings and sought answers to the appropriate questions, modified their brazier to summon Efreet.

Agenda: They are sages, librarians, arsonists, book-burners, witch hunters, and simple fire-worshipers: as diverse a group of Patriarchs as possible with no central goal other than their intention to take control of various churches and employ those faiths to their own ends.
Membership: Bishops (7th level cleric) might make discrete enquiries as to the cult but the cult will not allow anyone less than Patriarch (9th level cleric) to join their ranks.

The Wiren
The Wiren are an otherplanar race superficially similar to beholders in that they are a large dark organic sphere except they have two eyes rather than one and numerous fifty feet long tentacles rather than eyestalks. Wiren can puppet a victim using their tentacles, or infect the victim with parasites that provide the victim with greater physical strength.

Agenda: The Wiren have been employing contact other plane and commune magic to learn of other planes to invade and explore and downright infest. They desire power, territory, slaves, food, magic... if your PCs/NPCs have it and the Wiren does not, they will begin a plan to take it.
Membership: They are likely Clerics following some other-planar horror or Magic-users. They can be summoned during a conjure elemental spell simply because they are exploring the planes and are notorious for attatching themselves to an elemental.