The Possessed

There is no summon item for her. She belongs to the roster that fills the sky once a Solar Eclipse starts, so she shows up in daylight while the event runs and stops appearing the moment it ends.

She moves like a spider despite looking nothing like one. Put a background wall behind her and she runs straight up it, quicker than she manages on flat ground and impossible to shove while climbing. Out in the open she is an ordinary runner. Near a base, or any arena you bothered to wall in, she goes over the top of your defenses and drops on your head.

• Take the eclipse somewhere with open sky behind you. Bare platforms keep her on the floor where she is manageable.
• Knockback only lands while she is grounded, so do not build a plan around pushing her back.
• Kill her before she reaches a wall, not after.

The design points straight at The Exorcist. Her bestiary line also settles what she is: a Terrarian girl with something else driving, rather than a monster in her own right.