Chaos structures.

Medusa with a deadly gaze is a killer, especially if the hero has the Elven King's Bow and the "cat reaction" spell. It's simply a good fighter, and "terror" is a nice addition. As for Chaos Devils, no comment...

And if the hero also has "accuracy":D

Well, if we're talking about the topic, Ifrits + Chaos Devils + a bunch of heroes, mostly Chaos mages throwing fireballs, chokes, etc.:)

By the way, I think someone mentioned the high cost of building Chaos Devils somewhere; maybe I'm getting blind, or something changed in the same EC, but it seems like their lair still costs the same 14k + resources, and it's as "expensive" as any other dwelling:)

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(or even better when you have 2 castles and can take both)

And even better, 2 Chaos Devils per week:)