Atum Sum is a prodigiously skilled Chaos Sorcerer of the Thousand Sons Traitor Legion who has been active in the Jericho Reach. Indeed, he is even known to the Inquisition as an acolyte of Ahriman, one of the Legion's greatest sorcerers. He also bears the Mark of Tzeentch, carved beneath the flesh of his cranium.
Sum is steeped in the power of the Warp, and a known master of the daemonic. It is said that the daemonic servants of Tzeentch attend him as lesser men command the loyalty of stooping servants, though the veracity of this claim seems unlikely to those versed in the ways of the God of Change.
Furthermore, he appears to have an uncanny mastery of the ebb and flow of possibility, manipulating the shifting tides of the Warp to appear at any time and place he desires. It is doubtless true that Atum Sum is a dire threat to the Imperium, for his appearance is often seen to precipitate daemonic incursions of the most destructive kind.
Needless to say, Atum Sum's appearance in the Jericho Reach is a cause for dread, and several groups within the Inquisition have already set in motion plans to discern his purpose and if possible, oppose it.
To date, the sorcerer has been encountered in the warzones surrounding the Hadex Anomaly, leading some to suspect that he is involved in some grand scheme to utilise the weird energies streaming from it.
Some have even claimed that Atum Sum has conversed with the Anomaly, or with some vast, unknowable intelligence deep inside and beyond it.
Whatever the truth, several Inquisitor Lords of the Ordo Chronos were known to be mustering a coalition to stymie his plans when they disappeared, suggesting that his powers are an order of magnitude greater than any had dared believe.
Because Atum Sum has the ability to perceive the ebb and flow of the Warp, he can predict and to a degree manipulate the chaotic passage of time within it.
He can use this power to disrupt the normal process of cause and effect in realspace, seemingly cheating death as he explores one branch of probability to its ultimate conclusion, before backtracking to a previous branch of a possible outcome.
To this end, his foes find that time and again they believe Atum Sum to have been killed, only for him to reappear at another point in space and time to continue his unknowable mission.
Further compounding the issue is his mastery of a power known as "Warp Time." To those who know it, the timeless nature of the Warp grants them the means to defy the normal limits of time, hastening his movements and reflexes such that his enemies appear to be moving in slow motion.
This manifests as the sorcerer surrounding himself with a field of dimensional instability, warping the passage of time. The greater his mastery, the more devastating his combat abilities, speed, and aversion to harm.
- Deathwatch: First Founding (RPG), pg. 88