the actual boss arena of the plains looks like a giant skeleton hand. You know the standard plains rock pillars? That size fingers, or 5 of those pillars in one spot.

the boss stones are sometimes in villages and sometimes in little shrines. these are way too rare.

if annoyed enough, there is a map online that can show you your world from its seed or file and mark the locations of both stones and alter.

Thanks I think I'll check it out. I agree they are too rare, the game was really smooth in term of progression until the plains.