With the new ult transporting them into a ‘death realm’, there are so many skin universes that can fit this so well as well as old maps
Dark Star Morde - Transporting them into the dark Star real, similar to the map the Dark Star game mode used.
Blood Moon - Transporting them into a Bloodmoon realm, similar to the Bloodmoon map, but more elderich, horror like.
Arcade/Battle Boss - Pixelated/corrupted map of summoners rift
Sandlike theme - creating a sandstorm around the player and desert like textures, similar to crystal scar
There are so many ways making his ult look even cooler, and if they add an audience around the death realm in Pentakill Mordekaiser, then they can go all out with the effects of the ult in any of his new skins.
Fun fact: The ultimate is reminiscent of a scrapped Fiora ultimate, where she would teleport her and a target to the baron pit and 1v1.