Your body and soul are bound to a fiendish plane of existence. You have disadvantage on death saving throws as the plane attempts to pull you back to it. If you would be killed, your soul has been dragged back to a plane of existence of the GM’s choice. This plane is your new home, and you become an NPC under the GM’s control.
If you are on the fiendish plane responsible for your transformation, this flaw has no effect.