When a champion holds the red buff he deals some damage to the enemy after he auto attacks him. What type of damage does red buff deals? Magic Damage? Attack Damage? Or none of those?
Red buff applies a 8%/16%/24% slow (5%/10%/15% for ranged) and deals 8 + (2 x Champion Level) true damage every tick for 2 ticks (so basically it's from 20 damage level 1 to 88 damage level 18). As with all true damage, it is not affected by any damage reductions, however you can cleanse the debuff with a Quicksilver sash (Or Scimitar).