Mordekaiser's Q - Obliterate does not work properly during his R - Realm of Death
When Mordekaiser casts his Q right before his R's duration is about to end, it does no damage. My guess as to why this happens is because it seems that casting Q creates a hitbox on the ground that deals the damage, so when you cast it in the death realm, the damage hitbox is created in the death realm and not in the real world. This is probably not intended as casting Mordekaiser's E - Death's Grasp right before his R's duration still allows it to drag enemies properly.