The Ordeen is an Old God that appears in Book 6 (Clash of The Demons). It resides in a citadel, the Ord, that appears every seven years.
The Ordeen is depicted as a woman while dormant, but transforms into a flaming salamander while battling with Mam.
Thomas gives a cup of his blood. This grants him access to the chamber where the Ordeen resides. Mam enters from another entrance by smashing the portal. In Book 2, the Bane said that 'the moon shows the full truth of things'. Thomas sees Mam changing into her final Lamia form when the moon passes.
Thomas stalls for time when the Ordeen awakes, however the Ordeen transforms into a flaming salamander after Thomas uses his silver chain. Mam, in her Lamia form, fights the Ordeen while, Thomas, Gregory and Alice escape the Ord. On the way out of the ord bill stops to fight off some fire elementals and when the ord is engulfed in fire he is still inside.