An OC of mine.
Here is the story:
The demon realm is surrounded by a wall of spiritual fire that will burn absolutely any materials it touch to ashes, even high ranked fire demons can't stand this flame except one - the Fire Lord, also referred to as Hell Bird, the Phoenix.
Her immortal body is made entirely of the same spiritual flame that makes up the barrier between demon realm and mortal realm. She is the only one that can physically cross the barrier to human world, nothing can damage her and it also gains her the title of the strongest demon ever. But because her body are made of hell fire, it bring crucial pain to anyone she touches, so she is feared and very lonely, which results in her various mental weaknesses. Despite all of that, she is very caring and gentle in nature.
Most demons are natural soul travelers so they simply leave their corporal bodies behind and enter human world. There they must possess a human or animal or else they are basically powerless. but the mortal bodies are not compatible and limit their abilities so much that humans can fight equally to demons for centuries despite the immense difference in power. Demons are eager to find a way to physically cross the barrier and Phoenix is the key.