This is a major character, antagonist, on the first campaign on the Trikumai worlds, Stolen Dreams. He has not shown himself, but is cited as the main antagonist on the series of adventures. There are two characters named Mummy King, the older and the new Lord of Darkness, but the older is tricked by the First Raja of Prislýpea - also known as Principia - the land of djines, while the other survives the first story, and a major character marries him.
There is no real clue on the Mummy's powers, on this first campaign, but it's assumed he's immortal.
A main character deals with Darkness, as being one creature of Dark, but his efforts not to bow nor get down to his knees while the Dark Lord comes show he's in fact a heroe, or at least someone not disposed to follow any Lord - as he himself has said during the play. Krapulas, a daemon-like name to those from the Hell plane, are said not to follow this antagonist, but Trisebus, or Paramellechus, or the other main Daemon God unnamed, and so not some dark sidhas, the standard name for elfic heritage, they also don't follow orders from the Mummy.
The onlly appearance from the Mummy King is in fact a nightmare, which is a creature which assumes the form of you worst nightmares; it's Lena's most terrfying fear.
So, the play ends while this vilain still control his forces of dark sidhas, mentioned as one of three races, with no relation to each other. The Mummy's sidhas are those on the Dark Forest, in the border of the Feudal Federation with the west.
The west is the Mummy's most powerful enemy, where the monotheist empire of Trappke is experimenting the forces of exorcism, invocation of light and banishing of dark. It's said the symbols and prays they hold can turn away the forces of Evil - which would be their enemies. Not an example is given. The play shows in fact only a story under the viewpoint of the Royal Tachiza empire's main characters, even if two main heroes are actually from Nine Realms, and Prislýpea.
Another main character is taken to the moment when the Dark Lord is given life, probably by the Godess of Time and Magic, Ambar.
The Old Mummy King is said to be Queen Tachiza's enemy, about 1,500 years ago. They engaged magic war, and that seems the reason behind the immense Magic Faul - dead or savage magic area - which is centered on today's Royal Tachiza empire, the same place Krunnke's Brother comes to life again, and it looks to be also the same place Toriàn had put the Bumpper, 14 years ago.
See Death of Time for more on how the Old Mummy King died.
While the Mummy King is the concern of most of the population, the main characters in the play are more interested in the investigation of the Enemy.
This Enemy also seems to get the Mummy's attention, and the new Mummy King seems not to take action with this other vilain in mind, but yes, taking his plan ahead of time, not the reverse. It means, the Mummy doesn't look like considering the other vilain's action as opposition. That sill needs clearing, during the Final Version of the first campaign, but the Raýla plotline promise to show more about it.
He's still unalive, by the End.