It's just a better, more powerful version of the default dead thrall spell. I decided to make this just because the default couldn't get me a Valkynaz thrall. Not sure if this is related to this mod but the twin souls perk stopped working for me after making this and using it. Also, sorry if this doesn't work well for you because it's my first mod using the CK (any tips would be useful in the comments!).

The actual spellbook can be bought from any vendor who sells conjuration spells (e.g. Phinis Gestor in the College of Winterhold). It is easy to tell this apart from the default spell as it has (PWR) after dead thrall, to indicate its increased power.

If you don't want to buy it you can also add it through the console, just type "help "PWR" 4" and that should give you its ID, or if you don't care about a messy inventory just type "psb" and that should just add it to your magic menu.

Check out my other mod: http://www.nexusmods.com/skyrimspecialedition/mods/9416/?