I was brainstorming ways to use two ancient ones and hit upon this idea, one is the Main ancient one and one is the Other. Here is the basic rundown:
- The Main acts in all respects like an ancient one, gaining doom tokens and waking up, has 'worshipers' and 'stirs in his slumber'
- The Other hinders the investigators, but they can also draw on his power for help
- Drawing on the Other's power adds doom to his track, and some miscellaneous events can also add doom to his track
- Any turn that the Other gains doom tokens, its 'stir in slumber' and 'worshipers' abilities apply
- If the Other wakes up, the players must fight him to reset his doom track to half, and return him to sleep, then play continues
- If the Main wakes up, the players start the final battle as normal
So you need to write a bit of custom content for each ancient one you wish to use as an other, but that's about it. For example, consider using Cthulhu as the main and Cthuga as the other. The investigators get reduce San/Stam and scary cultists as normal, plus someone designs the following custom elements for using Cthugha as the Other.
So basically, the players are playing against Cthulhu as normal, but:
- When a weather mythos card comes up, Cthugha gets a doom token and his abilities activate for a turn
- Investigators can call Cthugha's fire by paying sanity, adding to his doom track, and suffering his abilities for a turn
- The game is harder because Cthugha occasionally causes hot weather and buffs fire vampires, even if the players don't use him
- The game is easier because the players can use Cthugha to roast monsters
- If they players aren't careful, Cthugha wakes up and needs to be defeated before the game can continue
I think this variant could be a cool way to use the less 'fun' ancient ones with short doom tracks and punishing doom-token adding abilities, such as Yig.