In the Object Editor I can see that many Abilities have some integer ( Spell - Tranquility - Hit Points Restored: 20) that defines their effect. In some maps I downloaded I noticed that this integer was set to negative (Spell - Tranquility - Hit Points Restored: -20). How can this integer be set to negative? Is it possible at all?
It is possible, Tides of Blood uses it a lot (ex. Bleed = negative Heal; Quicksand Aura = negative Endurance Aura). Anyways, this is how you make negative values possible in the editor:
File -> Preferences -> [x]Allow negative real values in the Object Editor.