Deadly Sphere

Nothing summons one directly. They arrive as part of the Solar Eclipse roster and only once Plantera is dead, so an eclipse that rolls earlier in Hardmode will never produce them.

It drifts toward you through terrain, ignoring walls, until it decides you are close enough. Then it grows spikes, starts spinning, and charges. The important part is that the phasing stops the instant the charging starts. An attacking sphere has to travel around blocks like anything else, and since it wants to sit directly above you and dive, a roof removes it from the fight completely.

Very little of your damage gets through. Weapons that win on hit count do close to nothing, and if each individual hit is small enough, the lifesteal from Vampire Knives or a Spectre Hood returns no health at all, which quietly turns a survivability build into a stalemate. Bring something that hits hard once. It is also small and quick, so minions and homing shots land far more reliably than aim does.