Water Bolt Mimic

There is no summoning item. It spawns like any other dungeon enemy with one extra condition: there has to be a placed book near the tile the game picked, and that book has to be far enough off your screen both across and above or below you. The mimic then materializes in the same space the book occupies. Find no book in range and it does not spawn at all. Two things follow from that. Stripping the shelves as you clear removes them from the dungeon entirely, and the book does not need a shelf or dungeon walls behind it, so laying your own books at the far end of a farm gives them somewhere to appear. Several can stack up on a single book.

Every other ambush in the game reacts to you. A Mimic or a Nymph breaks cover when you get close. This one does not care how near you stand and will sit still forever until something damages it. The cursor is the test: a real book gives you a name and an icon, this gives you nothing back. A book hanging in midair with no block beneath it is the other giveaway, because breaking the support does not disturb the disguise. It deals contact damage the whole time it is pretending, so most people meet one by walking into it.

The disguise ends the instant it takes a hit, and what comes out moves like a Cursed Skull, drifting through walls as though they were not there. Doorways and corners are not cover. It rotates unpredictably between three things: lobbing Water Spheres that also ignore terrain, backing off at speed when you close the gap, and charging hard when you give it room. The spheres can be shot down, which is the part to build around, so anything that fills the space in front of you answers both the ammunition and the charge. Chasing it when it retreats is the mistake, since you drag the rest of the room along with you.

It can drop the Water Bolt. That matters most on a world where generation gave the dungeon no copy of the book, which does happen, and where the mimic is then your only route to the best pre-hardmode magic weapon there is. The odds are poor, so treat it as a bonus for clearing rather than a farm, unless you have deliberately seeded books to make them show up.