The Psycho is a Hardmode, post-Planteraenemy that spawns during the Solar Eclipseevent. The Psycho will initially spawn in a nearly invisible, stealthed state, waiting for the player to damage it or draw near before it fully reveals itself and starts attacking. An attack made by the Psycho while in stealth will deal significantly more damage than an ordinary hit, similar to the function of the Psycho Knife it drops.
The Psycho is a reference to Michael Myers from the 1978 film Halloween, though the sprite more closely resembles Jason Voorhees from the Friday The 13th series.
For some reason, despite following the Fighter AI, the Psycho cannot open doors, nor does it interact with them.
The Bestiary entry for the Psycho: "This masked killer's quiet demeanor and cunning ability to hunt allows him to ambush his victims without being detected."