Soul Edge, its shards, or the Evil Seed. In Soulcalibur Legends, Malfested were referred to as Guardians and Evils. Malfested are driven to serve Soul Edge. A feature of some Malfested, most notably Zasalamel during the time he wielded Soul Edge, Nightmare, Charade, Night Terror, Necrid, and Pyrrha Ω, have a deformed arm, although there are exceptions, like Cervantes and Tira.
The term "malfested" was a localization choice to replace the original Japanese term "Evil (s)".