Sebastion Lort
The Caller
Anise
Master Necromancer from Ilinalta's Deep
Dremora Kynreeve from the Black Star
Drelas from Drelas' Cabin
Master Vampire from Haemar's Shame
Vampire Mistwalker from Haemar's Shame
For each of these, I put them in Breezehome, then fast traveled around Skyrim for at least 7 days. Upon returning they were all gone. So far, the only body that has remained over 30 days was an Ascendant Necromancer from Ilinalta's Deep. I'm not exactly sure why when the other Necros from that dungeon disappeared. Could just be a glitch? It's the necromancer that is found by the water with the drop-down bridge.
I haven't gotten many others yet, but it's incredibly frustrating to find a character that you want to Thrall then have them disappear in a day when walking around. You can't trust putting your loot on any of them because they may just vanish!
I haven't really tested, as I've been collecting data for use in mods when the CK comes out, but I would think that any quest-related NPC that starts essential and then loses the tag during the quest would qualify. Not sure about Ulfric or General Tullius, though, as each only loses the tag at the end of the other's CW line.


