Granite Elemental

Granite caves are a small underground pocket biome, dark brick and polished floor, and this is one of two things living in them. No item summons it and there is no statue, so meeting one comes down to whether the world put a granite cave in your path. There is a depth catch too: the pocket can generate at two different layers, but the elementals only populate the deeper one, so a granite patch just below the surface will be empty of them. It floats rather than walks, so gaps and ledges do nothing to slow it.

In Expert, taking damage can make it drop to the ground and armor up, and for a couple of seconds after that nothing you throw at it lands. It is a reaction to being hit rather than a timer, so unlike the Granite Golem there is no rhythm to learn and no window you can plan a burst around. You just notice your damage stop registering and wait. In a narrow corridor with other things closing in, that stall costs more than it sounds like it should.

Fire is the wrong answer here. Both granite enemies ignore burning damage entirely, so a weapon that leans on it will feel far worse than its numbers suggest.

Only a single Granite Elemental exists at any moment, which caps how ugly a granite cave can get. It also means the live one is holding the slot shut, so if you are farming rather than passing through, killing it is what lets the next appear.

Breaking line of sight does not shake it. It navigates toward wherever you actually are, following you up out of a shaft instead of drifting into the wall where you used to be. It cannot pass through solid blocks any more, so genuinely sealing yourself in works, but an open cave system gives it a route and it will take it.

Granite Blocks are guaranteed, and it shares the Night Vision Helmet and Geode rolls with the Golem. If either is why you are down here, farm the Golem instead: there is a statue for that one and none for this.

Before enemies were given proper internal names, this one was filed under Grelimental, granite and elemental jammed together in the same style as the Golem being called Grolem. The 1.3 announcement trailer shows several of them on screen at once, footage from a build made before the one at a time cap existed.