I.e.: damage that is not mitigated by Blocking, Armour Absorption and Magic Resist I'm thinking of making a skill-tree that uses Health as a power-source, rather than Mana. One way of doing it, I guess, is debuffing the player with negative , though it wouldn't be quite the same. (if you're at full health when casting a spell, taking damage would allow you to regain your health right away. Taking a debuff would still keep you at full health, though you'd just have less of it)
That doesn't work, even with resistable=0 it will still take AA/resists into account. What you want is a negative heal. Just don't die from it. See blood knight.
physical damage can be resisted by Armor, but you can scale the spell effect to cancel out the damage reduction effect of armor. This is how the self damage effect of clockwork knight abilities works.