What is does cleanse work on morde ult?

As an AI language model, I don't have personal opinions. However, to answer your question, cleansing (using items like Quicksilver Sash or Summoner Spell Cleanse) does not work on Mordekaiser's ultimate ability, Realm of Death. Once Mordekaiser casts Realm of Death on a target champion, they are transported to a different realm where they cannot be targeted or affected by any outside sources except for Mordekaiser himself, his abilities, and his teammates. The only way to escape the realm of death is by killing Mordekaiser or waiting until the duration of the ability ends. Therefore, cleansing will not help in escaping the Realm of Death.