Demon Altar. Craft Strange Looking Eyes (10 lenses), Worm Bait (30 Vile Powder, 15 Rotten Chunks (may be wrong?) ), and I believe some high-tier armor may require use of the Altar to create as well. Not sure on that part. You can't move them.
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cthulu comes sometimes by himself, though it usually gives you a warning message first of 'you feel an evil presence watching you' or something along those lines. It says that if you break the ground it's on, it disappears, though when I tried I just couldn't break the ground.
Also, it's not used for crafting high-end items, that was in the alpha. Now it's only used for crafting the two items to summon bosses, suspicious looking eye and worm bait.
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