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If my emperor's champoin fights solo against a space marine tactical squad, and i issue a challenge, the enemy must choose a tactical squad member to figh solo versus my champion. Am I right, or does challenge only work against HQ's and seargents.
what about, say the cursed knights of the legion of the damned?
Probably the best test to determine if they are characters would be; "Did you pay points to upgrade them, and/or do they have access to the armory?"
Off the top of my head I can't think of any charcters that don't have access to the armory, whether they are ICs or just upgraded squad members, with the exception of Tyranids, but I don't have their codex with me at the moment.