Saracen Rue was one of the nine members of the Dead Men during the War. He met the others when he "attacked a camp singlehandedly" according to him, but Shudder corrects him, saying that he "tripped and rolled down the hill into enemy camp" and the others had to save him. Saracen grumpily replies that it was an aggressive roll.
At one point Saracen also dated Tanith Low. While Saracen was dating her, the two fought together in a battle. After the night was over, the two kissed under the moonlight, drenched in gore and pieces of brain. Tanith recalls it as a "night enjoyed by all".
He also participated in a mission in Siberia to assassinate the Butcher, a soldier so terrible his own wife and soldiers disliked him. Saracen seduced his wife and hid with her for three days until his ankle healed and went up against Baron Vengeous in Leeds, and the psycho Sisters, Cerys and Aspen in Norwich. He also seduced them, but not at the same time.
He fought in The Corpse Attack in Denmark and was present at Anton Shudder's Birthday.
In The Maleficent Seven, Saracen made an appearance as one of the six people that helped Dexter Vex search for the four God-Killer weapons. It was shown that almost nobody knew what his magic was - when asked, he simply replied that he "knows things".
He helped defeat Darquesse, but she crushed him with a wall. It is unknown if he is alive or dead.
Apart from being said that he was an Adept, all that has been said about Saracen's power is that he "knows things". He demonstrated this when, for example, he knew that there was a guard around a corner holding a machine pistol without seeing him or hearing him. Some fans have speculated he has x-ray vision of some kind.
When Derek Landy was asked about his sexuality, he responded that there was none when one was 400 years old. 
During a Q&A at a signing Derek Landy was asked what Saracen's power was, Derek stated that Saracen Rue was his name in the books. This would mean that because Saracen is essentially the author he would know things that no-one else would.