Rending gives the weapon the ability to make its opponent vulnerable to attacks of a certain specific element. The amount of vulnerability depends on the attack skill of the wielder. This effect does not stack with Life Magic Vulnerability spells. Rending properties can be added to loot weapons by imbuing them using Weapon Tinkering.
Rending can 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 Rending.