An unstable portal. Doesn't oroginate from my realm or the other...
The Unstable Portal is an uncommon Hardmode flying enemy that only spawns on the surface. It has high damage, but no defence. It flies towards the player, usually above them for a while, and slowly moves down to attack. When striking a player, the Unstable Portal has a chance to apply the Distorted debuff, making the player to float up and down randomly. It drops many things, supposedly from a different dimension.