That wildness also made Maugrim possible; and because Maugrim came from outside the Tapestry there is no thread in it with his name on it, Flidais explains, and so he cannot die.
The unbridled nature of Maugrim made him viable; because he originated from outside the realm of the Tapestry, there is no thread within its fabric with his name on it, according to Flidais, which means he cannot be killed.