User blog comment:NicknameMy/Mordekaiser, the real bruiser/@comment-7709681-20150202143257/@comment-24982989-20150202165204

What I believe they meant about R being uncounterable is because it having a 20% slow and 30% damage increase which lasts for 10 seconds WHILE draining away HP and GIVING Morde that 20% MS taken. If Morde and his target both have equal MS, that is effectively 40% slow for 10 seconds barring his additional slows.