Drakin

The Drakin belongs to the Old One's Army and nothing else. It never spawns on its own, and it never appears in the pre-Hardmode tier at all. The second tier holds it back until the event is well past halfway, while the third brings it out a wave earlier and then keeps sending it for the rest of the run. Its first appearance is usually the moment a defense built for goblins stops holding.

Two things make it awkward. It carries far more health than anything else the event throws at you, and knockback does not move it, so the usual trick of pinning an enemy while your sentries chew through it does not work. It walks through your line at a constant pace whatever you hit it with. While it walks, it spits a tight cluster of purple bolts on a short loop, and the spread covers enough width that strafing sideways will not clear it. Put vertical space or a solid block between you and the volley instead. Focus one down as soon as it appears rather than letting a second join it, because two overlapping barrages are what actually ends a crystal run.