Talk:Karthus/@comment-4641298-20120227152215/@comment-49.147.139.139-20120302152337

Voidstaff is all you need to reduce MR. Abyssal is more suitable for AP heroes that needs tankiness (like Morde). Or to boost damage of tanky champs relying on magic damage but don't want to go full AP build (like Maokai, and Malphite, plus giving them nice MR too).

Why don't you do that calculation with only a voidstaff and without the abyssal. The resulting MR should only be 12MR higher. Not worth it (unless you're planning to build some sort of super tanky karthus).