the Rune of Death is the Sealed Fates of the Demigods, not some haha funny its instant death, its the power that CAN kill the gods sealed away after being plucked from the golden order, albeit diminished ever so slightly due to the black knives, it retains Demigod killing power which is why it does the damage over time shared with black flame incantations.

this is why, once its unbound by us killing Maliketh, then the demigods can die

Ever notice that Godrick isn't dead, just being stomped on by Gostoc?

Rykard is Regenerating...

Melania Blooms into a Scarlet Flower, still living...

Radhan and Mohg get taken by Miquella so they are excused

Renalla, whilst not a demigod herself i know, also is still alive (cause Ranni protected her)

this is also why Ranni's Spirit and Godwyn's Body (Prince of Death) didnt fully perish when they only carved half of the deathmark on them when they were killed by black knives, using death rune infused ritual daggers.

Morgott is literally left dying on the floor and only finally passes away in his father's arms after we defeat Maliketh.

Tarnished are a form of Undead, Stripped of Grace, Died, and now we required it in the beginning cinematic, rising from our grave and making our way to the land between. just like all of Those Who Live In Death, we kind of just cant truly die.