Another problem (which actually makes Dominions more charming than other games) is that the game really steers away from stereotypical fantasy. I'm sure everyone has noticed that none of the Norse inspired nations have dwarves, because dwarves and elves are totally overdone from a fantasy perspective. I suspect that's also why there aren't as many dragons in the game as there could be.
I also think "fantasy" fantasy creatures (those with no mythological basis) are not in the game simply because they are out of scope. But I guess they'd be fine for brainstorming purposes. So here we go:
Four Horsemen of the Apocalypse
Dragon King Zahhak (From the Iranian national epic Shahnameh)
Wendigo (someone else mentioned Sasquatch, basically the same)
Giant Spider (Shelob from LoTR, Anansi, etc.)
Basilisk/Cockatrice/Manticore/Chimera (These are somewhat covered by the Cross Breeding spell)
Sandworm (From Dune)
Grendel and Mother (although they have no physical description in the poem)
Cerberus/Garm (Giant Dog or Underworld guardian figure)
Leviathan/Behemoth/Ziz (I know they're already in the game, but these are supposed to be end of the world type creatures)
Apologies if I copied anyone!
Tetris Champion!
Re: The (not really) Official Endgame Summon Threa
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