Sion vs Mordekaiser Top Matchup

Sion Top wins 46.2% of 1,998 Low Elo games against Mordekaiser Top, -2.2 points vs expected, 2-patch window 26.19-26.20.

Researched on LoL patch 26.20. Tips are written to hold at any rank; stats come from Low Elo ranked games up to patch 26.20.

Why Mordekaiser is favored

Mordekaiser can isolate Sion with Realm of Death(R) and steal part of his stats. If Sion is already charging Decimating Smash(Q), the cast stays in the original arena and cannot hit Mordekaiser in the Realm, so Sion can lose his main engage when Mordekaiser chooses to ult.

How to play as Sion

Pressure Mordekaiser before level 6; once Realm of Death(R) is available, keep the wave near your turret and avoid chasing him when his ultimate is ready.

If Mordekaiser starts Realm of Death(R) while you are charging Decimating Smash(Q), release the swing only for the wave: it will resolve in the original arena and miss him inside the Realm. Sion players also report that a well-timed Unstoppable Onslaught(R) can counter Mordekaiser's cast.

If Realm of Death(R) starts while Sion is charging Decimating Smash(Q), Sion's Q remains in the original arena and cannot hit Mordekaiser inside the Realm.

How to play as Mordekaiser

Avoid starting Realm of Death(R) into Sion's ready Unstoppable Onslaught(R); if he is already charging Decimating Smash(Q), use your ultimate to leave the swing behind in the original arena.

Sources

These notes come from r/DirtySionMains and LoL Wiki.