Based on an old RPG villain I ran, more for a heroic fantasy game, but I figured I could make it work as an odd great old one. Not playtested (I just came up with it last night, and frankly I haven't even played against all the default AOs yet!) and indeed not even actually complete at this point (heck, may be such a bad idea it should never be used, but I'm just playing around with Strange Eons for fun ). Don't have the image or Eon file at the moment, so apologies on that.
The character concerned had chaotic powers, and spread confusion and disorientation among his enemies. Therefore, the idea here was to make his powers strongly random.
Barganon, Chaos Lord
Combat mod: -3
Doom track: 10
Worshippers: Barganon aids randomly. On each Mythos Phase, monsters whose symbols show up gain -1 to their combat modifier and +1 to their toughness until the next Mythos phase. This also affects trophies.
Power: Chaos Theory: At each Mythos phase, before the Gate step, all gates with symbols matching one of the symbols on the card are discarded and a new gate is drawn for those locations. This does not impact trophies. Any explored markers for those gates are discarded unless the new gate has the same destination as the old. This does not add to the doom track and does not count as "closing" a gate for purposes of eliminating monsters. In addition, if any encounter tells the player to search the deck and pick a specific card, he draws the top card on that deck and keeps it instead (this does not impact the initial investigator setup).
Start of Battle: After discarding any items which must be discarded when the Ancient One awakens, count each investigator's total items by type (unique items, common items, and spells). Discard all items, shuffle the decks, and draw an equivalent number of each type of item.
Attack: Each Investigator rolls 1 die. The result determines what Barganon attacks for that investigator this round. 1: Stamina. 2: Sanity. 3: Clue Token. 4: Monster Trophy. 5: Gate Trophy. 6: Nothing. Roll a Luck +1 check to avoid damage, and on a failure, lose 1 of whatever was attacked. This check's modifier decreases by 1 each round, even if Barganon attacks Nothing that round.
The general idea with this was to make an Ancient One that was quite hard to plan against for battle purposes. Since he can attack almost anything, you either have to go in with a decent collection of everything or trust in luck. He only takes 1 token/trophy no matter what damage type he does as a bit of a mercy point, since this allows characters who can negate sanity or stamina damage to get lucky and ignore damage one round. The "nothing" result is in there for the same reason (sometimes chaos helps you), but could probably be replaced by a 1 Stamina + 1 Sanity result if someone feels this is too easy.
The pre-battle "item switch" idea is to make it impossible to absolutely trust that you'll have, for instance, the Sword of Glory, but isn't quite as bad as Ithaqua's "item drop" ability..and could actually even help you. (Chaos cannot be completely controlled by anyone, after all.)
The "gate" power is based off of Daoloth's, but with a different trigger. Rather than every gate being triggered to swap during a monster surge, gates matching the symbols on the card are swapped. This means that less gates will be swapped at one time, but probably more often, likely causing more than a few LiTaS effects.
The "no specific items" power I just added now. O_O Investigators should still draw their standard fixed items, but any card that otherwise tells them to draw a set item (such as "search for William Brinton") instead gives him a random one (whatever is atop the deck).
The short doom track was designed partially to make the final battle shorter if it happens, but also partially to increase the chance of awakening. Because the likelihood of the awakening diminishes on doom tracks of 12 or above (since they can't awaken just from doom track points due to gate openings unless you're closing the wrong gates), I kept it shorter.
Anyway, that's the general concept. I think I've probably overdone it on the chaotic aspects here, to be honest, and this may be really overcomplicated to play against, but I just liked playing around with the ideas.
(And yes, if anyone is wondering, the name is partially due to the fact that I was a huge Legend of Zelda fan when this character was made (I still am, of course).)