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?

Eeyup, that's it. Huh. Can those things spawn outside of the Crimson? I've been in the above ground area that is over the altar, and I haven't noticed any.

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.