drain health 100 for 1 second and weakness to magicka 100% for 5 seconds. since weakness to magicka stacks first hit deals 100 i think, next does 200, then 300, then 400, then 500 damage as long as you hit them within 5 seconds. or you could do a damage health so it stays down and just keep damaging them more but from what i understand drain health is cheaper but if it doesnt kill them they get it all back. thats why the weakness to magicka
but the weakness would help with damage health too. at least that's what ive heard. havnt used it yet but soon as my mage gets high enough im gonna test it out. also frenzying people and then turning invisible is a good way to clear a group.