Sleepy Octopod

Ogres carry it, and ogres only turn up in the second and third tiers of the Old One's Army. Tier two is the better farm despite being the easier event, because the drop is meaningfully more likely there than in tier three. That also puts a mechanical boss between you and the weapon, since that is what opens the higher tiers of the event in the first place.

Hold the button and the head whirls in a full circle around you without stopping. Anything standing close takes repeated hits simply for being there, and the heavy knockback stops it closing back in. Where an enemy stands decides how badly it suffers: either side of you is the worst place to be, directly underneath is the safest, and distance cuts the number of connections quickly.

After a moment of spinning the tip glows green, and the next time the head connects with a solid surface at the bottom of the rotation it sets off a tall column of damage worth several ordinary hits. It has to touch something. On flat ground that happens automatically, and platforms and planter boxes count, so a plain wooden arena floor is enough. On a mount that lifts you more than a few tiles the head never reaches the ground and you lose the secondary attack entirely unless you back into a wall. The column also has a dead zone right beside you, so an enemy pressed against your body is standing in the one spot it cannot reach.

Farming, not bosses. It is at its best in the underground Hallow, where most of what comes at you has to physically reach you and the constant rotation plus knockback simply prevents that. Take it into the underground corruption or crimson and it falls apart, because nothing about a spinning head stops a projectile. It also lands no knockback on the explosion, which is another reason it never quite works as a boss weapon. Godly and Demonic come out equal on average damage, so take whichever appears first.