Dr. Man Fly

Nothing summons him. He walks in on his own during a solar eclipse, and only in a world where Plantera is already dead, so the eclipses you fought earlier in Hardmode never contained him. Once the event closes at nightfall he stops appearing.

He is one of the few eclipse enemies that warns you first. Watch for the moment he plants his feet, because the flask is already on the way. It arcs rather than flying flat, so it drops short if you close the gap and sails over you if you break away in time. The glass landing is not the part that hurts. What spreads out from the impact is, and standing in it while you trade hits is where the health goes.

He moves slowly enough that you pick the ground. The danger is company: a post-Plantera eclipse never sends one thing at a time, and the flask that kills you is the one thrown while you are dodging a Mothron pass. Clear him on arrival instead of leaving him for last. His name points at the 1958 film The Fly.