No, it's not increase damage spell, this only help to overcome enemies resist like slow enemies down, or freeze, something to debuff or weaker enemies without damage, I could be wrong...

+2 spell mastery is maybe .5% damage depending on the mob I suppose. It's not a very good trait or buff to an item. I like the +10 damage robes much more than the +10 spell mastery robes for example. I do take the plus spell master per level trait on character creation though.

Spell Mastery is what decides if a spell is resisted or not.

When you cast a spell on an enemy that can be resisted, the game takes your Spell Mastery and deducts the target's Spell Resist. The number you get is the chance that your spell will not be resisted.

75 Spell Mastery vs 20 Spell Resist = 55% chance that your spell will not be resisted if it can be

Thanks, that makes sense. but does +2 literally mean 2 points or does it represent a multiple of some number?

For example would +2 raise the mastery in your example to 77, or would it raise it to 85 (where each +1 would equal a multiple of 5)

Thanks! I don't think I'm going to waste my time pursuing spell mastery adjustments/bonuses, if all you get is +1 here or +2 there.

Doesn't seem worth it.

Yeah, you can't waste 5 turns trying to cast blindness or shrink on a dragon with a 20% to get the spell through.

By then you should have a defender with 100% fire resist and big dodge, defence and HP.

Like this? http://stormworld.wikidot.com/reliquary#Vorpal_Sword

Bear in mind that there's the Brilliant trait that you can pick for your sov at creation, +2 Spell Mastery per level, not to be sniffed at.