Well, the Deep turned out not be as terrifying as I hoped
Congratulations to Dr Praetorious, whose sieging force reveals that he's been focused on the winning criterion from turn 1.
Despite losing their pretender to Vengeance of the Dead, the forces of Pythium easily overwhelmed the embarrassingly weak defense stationed in, and teleported to, R'lyeh. I blundered the previously turn by accidentally setting units to patrol instead of defend, but I don't think it would have made any difference.
Pythium has won the game. Thanks to everyone for playing.