XOL rune does not work for Necro. I have tried it with a number of runes and it simply does not work. My first set uses the rune that generates 150 offering when casting an ultimate (which then activates earthquake rune). As a necro that counts as another class skill which should then satisfy the requirements of XOL and then activate the rune connected to it. It does not matter if that rune requires 150 offering or more. The icon does not fill up and the skill does not activate.