A exceedingly powerful Necromancer, Anub'rekhan has become the pinnacle of undeath, achieving the status of Lich.
Little is known of his history or race. Every time a member of the party fell him they received a glimpse into his past revealing that he had necromantic powers as a child and was feared and scorned for them. This implies that his powers were innate not learned.
Undead Dragon Riders
Employ
The Lich has dragons under his control. Whether this is by some forceful manipulation or a deal struck between them is unknown. Either way the Lich's undead creatures ride these dragons.
When Arcturus was founding The Empire he struck a deal with the Lich to employ his revolting creatures. The price was that Arcturus would send the Lich a certain amount of corpses at regular intervals.
The Dragon Riders lend their services, for this price, and have turned the tide of many a battle in favor of The Empire.
Hanzo Hattori slayed one of the riders during the Battle of Thrones and it's dragon flew away.
Death
At some point Valerian became wary of these creatures and killed the Lich, making the Dragons a non-problem and replacing them with Clockwork Dragons (see Alexandra Mahone for more info.).
The sound the Lich makes when dying
Lieutenants
The Lich employs two people to help him with various tasks. The Red Elf and The Ghul (see Lich's Castle for more info.).
Resurrection
"What made you think I wanted him dead?" - Anub'rekhan regarding David.
Mal'Ganis raised Anub'rekhan from the dead (see Mal'Ganis for more info.). At his resurrection he was greeted with an apparently-dead David, his supposed killer, Mal'Ganis, and a few of his other servants. Even a necromancer is not immune to resurrection sickness, but certainly is knowledgeable on how to overcome it. The Lich absorbed all of his servants except for Mal'Ganis in order to gain enough power to get past some of the sickness, though he was not back to full power. He did not want to betray his weakness and left Suko alive mostly because he didn't want to stress himself unnecessarily. It was because of this weakened state that he was able to be deceived of David's status (see Setsuko for more info.).
When Setsuko claimed that she had done what he was unable to - kill David, Anub'rekhan scoffed. He reveled that he did not want David dead and that he was in fact not afraid of the creatures from David's world that everyone had been interacting with (the SCPs), but instead was weary about the 'other ones' who came across (it is now known that he was referring to the angels that 'The Author' brought over).
He left with Mal'Ganis to the Lin Kuei's cemetery and used much of his strength to raise many very powerful warriors back from the dead. He then traveled to his castle to find it refurbished into the SCP Foundation. He used his new army, and every enemy that fell, to defeat the guards posted outside the foundation. As he was planning on how to invade the inside of it David of all people showed up! The Lich waited to see what would happen before acting. He witnessed David leave back to his world in a strange machine and all the SCPs go back with him.
What he did then, and what he will do now is unknown.