Fire Rending gives the weapon the ability to make its opponent vulnerable to fire attacks. The amount of vulnerability depends on the attack skill of the wielder. This effect does not stack with Fire Vulnerability spells. Fire Rending can be added to loot weapons by applying a bag of Salvaged Red Garnet using Weapon Tinkering.
This can also be found on some quest reward weapons, though it is much more common for them to have Resistance Cleaving which is a fixed effect, instead of being skill based like Fire Rending.