Sapphiron was an agent of the blue dragonflight who had guarded a treasure hoard in the mountains of Northrend. After being slain by Arthas Menethil, the dragon was raised as a powerful frost wyrm and sent to the halls of Naxxramas, where he guards the inner sanctum of the citadel's dread master, Kel'Thuzad.
This gives Sapphiron magic resistance, similar to the resistance that level 6+ creeps have. Namely, you can't be Polymorphed, Possessed, etc, and all offensive spells have a shorter duration on the target (such as stuns, Ensnare, etc). It DOES NOT reduce damage dealt to the target, however. That’s an effect of hero armor, not hero magic resistance.
Gives Sapphiron increased resistance to spell damage.