The Dark Master
Grieve is the only one claiming to ever come in 'contact' with this creature. The Fire Lords as well, have had some speculation as to whether they were brought into existence by this supposed being as they were the first demons in existence and still do not know how they were brought to 'life' (they simply don't care). They do not believe in these speculations, and anyone who brings these thoughts to their attention are slaughtered mercilessly. The Dark Master is thought to be something like a 'deity'- however, it is known that gods do not exist, only beings of immense power, yet it is also known that nothing is all powerful. The creature is merely used as an excuse for all things currently unexplained in the realms of existence. If one were to believe in a 'God', the Dark Master would be the closest thing to one. Grieve is laughed at by the Fire Lords for claiming to have spoken with the Dark Master when he came into existence- as Grieve was the first Torture Demon created.