About suicidal dreams spell - I love flavor, but it might be tough to balance. Seems too powerful for low level casters, and if you have high level one near enemy city, you probably have more important things to do. But maybe it's possible to be castable from your city, affecting one city with OO anywhere in the world ? Something like Infernal worldspell, giving you choice of three or five best OO cities.
You have a point. How about this serial of synergistic spells.
To emulate that in Lovecraftian stories cults of the Overlords are acting outside the range of vision OO disciple units could use a Whiteout spell that works on coastal tiles instead on snow tiles. This way you don't have to lead war against the leader whose cities you want to affect with your spells.
Spell for all disciple units:
Cast time: 1 turn
Requires a coastal or ocean tile.
Grants the Hidden promotion to the caster
Disciple spell:
Cast time: 1 turn
Targets a city within 1 tile.
Effect: Spreads the OO religion. x% chance to remove the hidden promotion from the caster. If it is removed, declaration of war.
Priest spell:
Cast time: 1 turn
Targets a city within 1 tile.
Effect: 1 unhappiness that ends after one turn (similar to the spy task that spreads 8 unhappiness). Autocastable. x% chance to remove the hidden promotion from the caster. If it is removed, declaration of war.
Highpriest spell:
Cast time: 3 turns
Targets a city, that is determined by following values:
Overall city unhappiness
Present religion (in descending order): OO --> AV --> CoE --> Order --> WH --> RoK --> FoL --> Empyreon
Controller: All other leaders (if this is possible leaders that hate you --> leaders that like you) --> you
Effect: The city loses 3 population.
This means you have a High Priest that sits in one of your city pulling the strings behind the scenes. And an OO army with some disciples and priests that make sure the city next to them is the city that will be targeted by the Highpriest.
Concerning drowned: Has anyone read Song of Ice and Fire ? The worship of the Drowned God is a way I could imagine an OO initiation ritual.
In Song of Ice and Fire those who want to serve the Drowned God are in fact drowned by putting their head under water until they seem to be dead (in fact there is still a bit life in them). Then the priest gives the seemingly dead an artificial respiration. Some of the attendants survive the procedure, some of them die.
This brings me to another idea: The OO cult could have different levels of iniation. Each level risks killing the caster, but is also able to make it stronger.
You could emulate this by this spell:
Grip of the deep:
Unitrequirement: None
Techrequirement: Message from the Deep
Cost: x Gold
Deals 75% cold damage to the caster (this is an avarage value. Thus most will survive, but some will die). Upgrade caster to a zealot (if he is not a Cultist or a Stygian guard or a OO immortal). 50% chance that the caster gains the resistance to cold promotion. 100% to add the Grip of the Deep promotion.
Grip of the Deep
Effect: None.
100% chance to wear off at end of your turn. This promotion serves only to make sure that the spell Grip of the Deep has been casted.
Grip of Insanity:
Promotionrequirement: Grip of the Deep.
Techrequirement: Priesthood and Message from the Deep
Cost: x Gold.
Deals 75% mind damage to the caster. Upgrades the caster into a cultist (if he is not already a Stygian guard or a OO immortal). 50% chance that the caster gains the immunity to cold promotion. 50% chance that the caster gains the resistance to mind promotion. 100% chance that the caster gains the Grip of Insanity promotion. If the caster dies he will become a lunatic instead.
Concerning Cultists I imagine something different. Someone who wants to become a cultist first will be drowned again. Then he has to face his very own fears, a test of his mind that will decide if he will be sent into the asylum or the temple of his masters.
Grip of the Insanity
Effect: None. This promotion serves only to make sure that the spell Grip of Insanity has been casted.
100% chance to wear off at end of your turn.
After this the next step. The caster first will be drowned, then he will face his own fears, that will mix with images of the Overlords that more and more become real till they consume the body of the attendant and he becomes a Stygian guard. But only a few can control the new abilities they have gained and often their new form turns against the followers of the cult. I think Stygian Guards should be an almost perfect aspect of the physical form of the Overlords whilst only a small part of their mind still show in their guards. The full power of their mind will show in the immortal unit.
Glimpse from a world beyond:
Promotionrequirement: Grip of Insanity
Tech requirement: Fanaticism
Cost: x Gold.
Deals 75% chaos damage to the caster (or what new damage type do you consider best?). Upgrades the caster into a stygian guard (if he is not already an a OO immortal). 50% chance that the caster gains the immunity to mind promotion. 50% chance that the caster gains the resistance to chaos promotion. 100% chance that the caster gains the "Glimpse from a World beyond" promotion. If the caster dies he will become a barbarian Stygian Guard instead.
Glimpse from a World beyond:
Effect: None. This promotion serves only to make sure that the spell Glimpse from a World beyond has been casted.
100% chance to wear off at end of your turn.
In the last step the attendant faces the Overlords themselves, beingd beyond any human understanding. He opens them the deepest secrets of his soul and they open him their souls. He sees some of the greatest secrets of the universe, finally he understands some of their motives that make them intervene into the mortal interests and his soul and theirs become one until he comes back with a part of their soul within his soul. But as this experience is so powerful the danger that his own soul will be consumed is great. And if this happens endless screams spurt out from his mouth that let go all people people shout and kill and revell in joy. But this is nothing compared to the possibilities that await those who can keep a part from their souls alive.
Unity with the Overlords (Castable once per game)
Cast time: 10 turns
Promotionrequirement: Glimpse from a World beyond
Tech requirement: Divine Essence
Deals 75% spirit damage to the caster (or any damage type you consider appropriate). Upgrades the caster into an OO immortal (as Ahwaric said he wanted an OO unique unit for the immortal slot). 50% chance to gain immunity to chaos damage, 50% chance to gain resistance to spirit damage, 100% for unity with the Overlords, finally 20% chance for EVERY BENEFICIAL PROMOTION to be gained. The unit has gazed into the knowledge of the universe and suffered great perils to do so. If the unit dies in this process all cities suffer 100 riot turns (=10

) and all units have 50% chance to become insane.
Unity with the Overlords:
+5 mind-affinity. Every killed unit transforms into a Stygian Guard on your side.