The Vanquished Gods
During the Age of God Wars a multitude of pantheons battled on the Sister Continents, and while most deaths and casualties were of the mortal races, the gods too suffered. While some gods were slain battle (this was rare, due to their ability to return to their home worlds in the blink of an eye) more often they were driven from the Sister Continents, and their spiritual links to the mortal world severed. To defeat another god (or pantheon of gods) is in effect to make them "vanquished", but what does this mean?
In order for the gods to reach the Sister Continents during the Age of of God Wars it took the prayers of hundreds or even thousands of devoted followers, which could vary due to several factors that we won't go into here. The important thing to understand is that without followers and their spiritual links with their gods, the gods were (and are) powerless on the Sister Continents. Therefore, the surefire way to defeat another god or pantheon is genocide. Kill every last follower of a pantheon and that pantheon's connection to the mortal world is gone. Genocide is, of course, the most extreme method. Most wars centered on conversion, with the premise being new followers strengthened your own pantheon while weakening your opponent.
The gods who belonged to these marginalized pantheons (who may retain some minor connection to the Sister Continents) were called the Vanquished. In other cases, pantheons with few remaining followers could be shunned for a generation or two, and as the elders passed, the pantheon could in effect become powerless because they've been Forgotten.
So, this brings us to the phrase, defiled by the vanquished gods.
Most magic in the first thousand years after the Great Forgetting revolves around prayer and the gods, where the energy for magic is channeled directly along the spiritual tether between mortal and immortal for the prayerful to direct into a specific effect. There are, however, a few people born who discover they have the ability to manipulate the Elements without the assistance of the gods and prayer. Most of these folks find their abilities to be in manipulating Elemental Life, and in healing, and therefore are more-or-less tolerated, even if they might suffer varying degrees of suspicion or stigma.
A smaller number of these gifted people find their talents in other more dramatic Elements, such as Fire, and these unfortunates are labeled as Defiled by the Vanquished Gods. The magic they used is called feral magic. They are shunned, sent into the wilderness to die as children or forced to turn themselves over to local priests for a "spiritual cleansing" and are never seen again.
The source for the term defiled is found in the Book of Bedæ, where it is claimed that the vanquished or forgotten gods will seek to reconnect with the mortal world via prophets who will wield the powers of these gods. Without another explanation for these children, the church made the leap in logic early on that these people were the prophets of which the Tomes of Sôl spoke. They are considered heretics and dangerous to the church and the gods themselves, and therefore are sought out and killed.
Higher ups in the church, however, realize this is not true, but still these people are seen as a threat, as the Tomes of Sôl discusses sorcerers who wield magic without the aid of the gods. Generally speaking, these wielders of feral magic are weaker than priests, but in an age where the gods are weakened, the church fears their rise.
In western Earth history, the defiled might be comparable to witches in historical Christian terms, people who've struck a deal with Satan.