How does it works?
A: Removing a hero's inate 25% magic resistance, leaving them with 0% magic reduction.
B: Amplifying the spell damage by 25%, i.e. 100 damage spell now does 125 damage, then it is reduced by the hero's magic resistance.
C: Reducing the current hero's magic resistance by 25%. Example: Heroes using Hood/Pipe or heroes with skills that gives magic resistance.
D: None of the above
Best Answer
If you use Veil, it "reduces" magic resistance by 25%, but remember since magic damage is calculated multiplicitavely, the enemy will still have 6% magic resistance left over. So you will deal 19% additional magic damage to a hero that had 25% base magic resistance.
Also remember Magic Resistance reduction technically does not exist, but if you hover your mouse over a hero's stats it'll say they've been reduced to 6% when in reality the magic damage is actually amplified to mimic that.
So in short, Magic Damage x 1.25 but the target still has 6% resist if he was a 25% resist hero.
stolen from reddit http://www.reddit.com/r/DotA2/comments/1vbiya/mechanics_how_does_veil_of_discord_amplify_magic/