Vampire

It appears during a Solar Eclipse and at no other time. There is no summoning item, nothing spawns it on an ordinary day, and when the eclipse ends any survivors run for it at dawn the way the rest of the event roster does.

It picks a shape based on where you are standing. Get above it, or open up any real distance, and it flips into a bat that crosses the gap at speed and changes back into the humanoid the moment it arrives. That transition is what catches people. You back off to make room and it is already on you before the retreat is finished, in a form that reaches places the walking version cannot.

Contact leaves you Bleeding, which shuts off natural regeneration, so trying to heal through a bad exchange works worse than usual. Expert roughly doubles how long that bleed lasts and Master stretches it further still, which turns an annoyance into a genuine drain when two or three are working on you at once.

An eclipse darkens the whole sky, and a small dark bat against a dark sky is genuinely hard to pick out until it is on top of you. A Hunter Potion is the single biggest improvement available here. Failing that, light the ground you intend to hold, or carry a light pet like the Wisp in a Bottle so a glow travels with you. A Shine Potion does much the same job on a budget.

The Stake Launcher hits it for ten times its normal damage, in both forms. That is enough to reduce the tankiest thing in the eclipse to a formality, and it earns a hotbar slot for the duration of the event even on a character who never otherwise picks up a gun.

Nearly every eclipse enemy is a classic horror film reference and this one is the 1931 Dracula. The Stake Launcher bonus follows the same thread out of folklore, where a stake through the heart is the only thing that finishes a vampire for good. Sunlight, oddly, does nothing to them beyond making them flee. Sand Sharks borrow its idle noise for their jump attack.