How does Veil of Discord damage amplification works?
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/