I'm about 250 turns into my game. All surviving AI report they haven't met Altar in the foreign relations screen. I enabled cheats to remove the fog of war to see if they'd been placed on an island or something, but actually Altar was no where to be found.

I would guess from this that the game never placed Altar's sovereign on the board when the game started? Maybe he was wiped out by monsters early in the game, but in that case wouldn't it trigger the popup that Altar had been defeated? Anyway, just thought I should report this mystery.

Let me know if you need more info.