Kobold

Kobolds exist only inside the Old One's Army. They start early in a tier two defense and stay in rotation for most of it, and tier three fields them from its second wave on. Nothing summons one and none spawn otherwise.

It is a walking bomb with a tell. Get near it, or let it get near the Eternia Crystal, and it throws an exclamation mark over its head, lights up, and sprints screaming at whichever target is closer. Arrival kills it and sets off a blast about the size of a grenade. Your arena survives that. You might not, if three come together. The emote is the only warning you get, and no other enemy in the game gives one.

Standard crowd control does very little here. Lava does not touch it, teleporters ignore it, and traps only land half their damage, so a trap corridor that shreds everything else lets kobolds walk through. Blocking the path is unreliable too: if it cannot reach a target it turns ethereal, floats upward, and passes through your walls to try again. Kill them at range before a fuse reaches the crystal, and expect the banner to take several full events.