Board Thread:Skyrim/@comment-108.248.201.126-20140428193109/@comment-29458028-20140621024708

Durnehviir is considered a summon, by game files, so probably they create a new entity every summon. You can actually summon durnehviir in the soul cairn beside the soul cairn durnehviir, providing more evidence that they disabled the original and the shout makes copies. So Marked for Death would not work long term. I am not sure of this, though, so you might take a save that you don't want anymore and MfD durnehviir to check

(also, anyone would prefer if Durnehviir used the purple flames he gives of when he dies as dragonfire? it would make him more unique)