I've encountered something a little... odd in my mine. I've tried consulting the wiki, but to no avail, and it does not resemble anything I have ever seen from LPs (although I am only familiar with TB's, which is apparently from an older version that looks different). It's a mass of purple, fleshy stuff, with what looks like eyeballs somewhere on top. The only thing that gets a reaction out of it is the hammer, and that just causes me damage (20 a swing, iirc). My first thought was that it might be a demon altar, but the area in question is not corrupted. Does anyone have any idea what this thing might be?

Altars, both corruption and crimson tend to appear on a 'wherever they please' basis during generation. They're not renewable, however. You can be deep in a cave, far from either biome and there pulsing in the darkness can be one.

Save for the actual surface, of course, barring freak events.