Insanity Devils

Insanity Fiends are a series of FIRE Fiend type monsters who revolve around summoning themselves when a specific condition is met with activated effects that are mandatory. These summoning conditions tend to activate when a card leaves the field or a specific phase is entered when a condition is met

Appearance
All members have the appearance of vaguely humanoid creatures who's main body is Black with multiple shades of red and orange higher up on their bodies (usually their heads). Their name partly gives away their appearance. This is best shown in Skeletal, Malformed, Lurking

Their name also hints at a state of mind or phobia people have such as Insane, Wailing, Feral alluding to the concept of Insanity