William de Mornay was a mercenary who killed many people across Europe in battle.
When a Dracorex from the Cretaceous enters a Medieval forest via an anomaly, the people of the nearby town send de Mornay to help, as they believe it to be a dragon, and de Mornay, hoping to gain holy redemption by killing the creature, accepts. He chases the Dracorex through another anomaly into London, 2009, and mistakes the place for Hell. He "visits" a bar, but ends up starting a fight (after seeing a bar patron tattooed with the number '666'). During his journey, William reaches his own grave and thinks he has gone to hell after death. Determined to escape, he believes that only by slaying 'the dragon' will he be able to reach Heaven and suspects the ARC team to be demons, culminating in a vicious sword fight with Danny Quinn.
Sarah Page stops him killing Danny at the last second, and she shows William his grave and calms him down. Sarah also tells him that he will marry a local noblewoman, Elizabeth Lionel and have children, but also lets slip that she knows where the "dragon" is. She is then forced to show him to the Dracorex. When Abby Maitland is willing to sacrifice herself for the Dracorex, William drops his sword and returns to his home era, giving Sarah a coin as a parting gift. On his return, he is proclaimed a hero by the villagers and receives a kiss from his wife-to-be.
- The look of his armour, and the fact that he spoke in Early Modern English, would suggest he came from the late 15th to early 16th Century.