Beguiling Lyre

A faun walks along behind you. When you get too far ahead of it, or jump somewhere it cannot reach on foot, it lifts off and flies until it has caught up, then drops back to the ground and resumes walking. That is the entire feature set. It deals no damage, it lights nothing, and no recipe, achievement or progression step anywhere in the game cares whether you own one.

The one real cost is a buff slot. A pet holds its slot for as long as it is out, which matters during a boss fight where you are already running several potions and want the row clear.

It comes out of crates, not off a boss, and that changes how you should think about getting it. Nobody sets up a farm for it. It sits in the middle tier of the fishing crates, one version before hardmode and one after, so the players who own a faun are almost always the players who were already pulling hundreds of crates for ore, potions and accessories. If you skip fishing, you can finish a world without ever seeing it.

For most of the game history this was called Tartar Sauce and it summoned a small minotaur instead. The name, the pet and the odd question in its tooltip all came from a song by a YouTuber who later became someone Re-Logic preferred not to be tied to. The 1.4.5 update replaced all of it at once: new name, new sprite, new buff, new pet. Older videos and guides still point at an item that no longer exists, so if you are hunting for Tartar Sauce in a current world, this is the thing you are looking for.

The minotaur it used to summon, red loincloth and hammer included, looked a lot like the Maulotaur from Heretic and its sequel Hexen, a monster you fight in one game and get to summon on your own side in the other. The replacement reaches further back. A faun is the Roman woodland spirit, roughly the same idea as the Greek satyr, half man and half goat, and the lyre in the item name is the instrument that calls one out of the woods.