I would say Shado but he is really an already existing character whoms real identity is secret so... And he is Firelord Surdor.
Now, I'm not a MOCist but here's a rough idea of how he looks in one of his final stages:
So I haven't really finished making his storyline yet but basically he is like a demon-like species and the second to last of his kind (he killed the rest). He has an army of fire/lava Rahi, mutated Ta-/Onu-/Po-Matoran and warriors of a species native to his home island. He has a mental link with all of them and they are basically brain washed.
His powers include elemental control of fire and stone, as well as partial , non-elemental control over sand, earth, plasma, iron and air. He also has extreme heat resistance. Another one of his powers, which makes him close to invincible is reincarnation.
The way it works is that when he dies his spirit leaves his body and is transferred to a close by drone made to be able to contain a demon. When he finds a new suitable body, his spirit transfers over to it. This body has to be dead, be strong enough to contain his energy (thus a Matoran or Turaga does not work), be sapient and must be an MU species. When he reincarnates his body transforms into energy that he takes with him into the new body, making his new form even more powerful. When he enters a body it gets mutated by the energies, healing most kinds of wounds including a stab through the chest or a missing limb.
If he was to transfer into a headless being he would die however. If the drone carrying his soul dies he would die aswell, which are his only known weaknesses.
The regeneration effects of the mutation does not give him regeneration abilities after mutating. And he can not leave his body at will. It does happen that he kills himself to get out of situations, fool his enemies or just to regenerates. After all, he'll get stronger anyways.
He has a fire staff and a sword as tools. They are practically indestroyable and will teleport to him when he reincarnates.
His home island used to have Ta-, Onu- and Po-Matoran and is cut off from the rest of the MU. He was freed from the island after the battle between Teridax and Mata Nui and now wants to rule 'the world' (or whats left after burning it).
Now I did mention there being more of his kind. While they are not any danger to him, they hold the knowledge about how to defeat him which is his reason to killing them. Tje one that is alive seeks one thing: revenge for his kind by killing Surdor.