Well, the effects I tried so far did not accept negative values, I tried to make an engine, that would speed up that character, but couldn't get it to work
(and something else too, but can't remember right now). And I think Attacker takes damage means those that attack you. But perhaps it is possible to have a spell with target type self and a damage effect?
edit : just tried out a negative 'life' effect, the game just reverted it to "+0 life"