Clownface

History
The villain that would come to be known as Clownface originally began as a stray piece of Clayface's body that became unattached and gained enough sentience to morph itself into a mute old man. This man was found and taken to Arkham Manor because of his unresponsiveness. Bruce Wayne who was undercover as Jack Shaw in the manor, was able to recognize the man as a portion of Clayface but avoided blowing his own cover to alert the guards.

The Joker, undercover, too, as the Arkham attendant Eric Border, shed his disguise and secretly infected this portion of Clayface with Joker Venom, morphing it into an entirely separate killing machine he called Clownface.

Clownface wrecked havoc in the manor, killing multiple people, before being subdued by Batman and Victor Fries. Frozen solid thanks to Fries' help, Clownface was stored in the manor's freezer until he was to be collected by the guards.

When Jeremiah Arkham went insane and released all of the prisoners in the manor, pretending he was the Joker, Clownface escaped and got into a fight with the real Clayface, who was attempting to escape the Joker-infected asylum.