Hello,

I received an item in Skyrim that does 20 points of magicka damage. Does this mean that it deals 20 extra damage in the form of magicka, or does that mean that it drains 20 magicka whenever I hit something with it?

It damages their magicka. They'll lose 20 points of magicka whenever they get hit with it.

Ok, thanks! Clears that up.