Back before Poppy's rework, her ultimate ability was **Diplomatic Immunity.** It was considered to be extremely powerful--and frustrating--because it made Poppy nearly invincible while she dived your back line and/or assassinated your carry. Mordekaiser's new ult testing on the PBE, **Realm of Death** , is very similar to this but with some important differences.
**Diplomatic Immunity** provided limited counterplay for the target, as Poppy would often simply ult the support and ignore damage from the targeted carry or enemy. However, this did not stop you from body blocking, healing and shielding her target, or even just running away for the duration. **Realm of Death** let's the target fight back, however the team has no agency at all in trying to prevent it or protect the target. Additionally, you as the target cannot run away easily, as the ult closes Mordekaiser and the target in an arena.
It should be considered that the game is very different now than it was then. There are several champions in the game that have similar "locked in a duel" effects--Camille's ult comes to mind. Hopefully this doesn't become a repeat of a frustrating dive character that can't be countered.