Arkhan the Black has served Nagash since before the Mortal Realms were young — a liche whose memory reaches back to a world that died and took its history with it. Where other lieutenants of the Great Necromancer scheme, resent, and test their chains, Arkhan simply continues, dry as dust and twice as patient. His will is so entangled with his master's that scholars debate whether Arkhan is Nagash's servant or merely Nagash's habit of delegating.
It was Arkhan who oversaw the great project that became the Ossiarch Bonereapers: the founding of the Mortisan orders, the doctrine of the soul-blend, the quiet vaults where the first Mortek phalanxes were poured. As Mortarch of Sacrament he is the empire's high priest and chief architect in one, riding to war on the dread abyssal Razarak when magic must be answered with older magic. The necroquake that shook the realms was Nagash's triumph, but the scaffolding beneath it was Arkhan's, as the scaffolding beneath most of Nagash's triumphs has always been.
Even Arkhan's destruction is treated, in the Ossiarch Empire, as a clerical matter rather than a tragedy — for those who know the Great Necromancer best observe that he has never yet permitted so essential a tool to stay lost. Death, after all, is the one door in all the realms that Nagash holds the keys to, and Arkhan has walked through it before.