Mage · Arcane · tier 2 · from level 15

Arcane Subtlety

Rank 2/2

Reduces your target's resistance to all your spells by 15 and reduces the threat caused by your Arcane spells by 30%.

Doesn't change the damage or healing of any Mage spell.

Averages, counting crits and misses, for a level 60 Mage with a typical weapon and no other gear or buffs, against a target your level. Worked out from the game's own data, the same way as the Foresight addon.