What is the Dead Thrall spell and how do I get it?

Solution 1:

The UESP (Unofficial Elder Scroll Pages) is the most comprehensive repository of information on Elder Scrolls games, Skyrim included. For questions like this, using the UESP's search feature almost always turns up the answer right away.

In this case, it has an article on Dead Thrall, which says in part:

Dead Thrall is a Master-level Conjuration Spell which raises a dead body permanently to fight for you.

It goes on to say that to obtain the spell you have to have a Conjuration skill of 90 and

complete the Conjuration Ritual Spell quest.

Solution 2:

The Dead Thrall spell is a master level conjuration spell unlocked after completing the Conjuration Ritual Spell quest. It animates almost any dead humanoid of up to level 40 permanently, i.e. until they die again, to fight for you.

To begin the ritual quest, talk to Phinis Gestor after reaching 90 conjuration. You do not learn the spell directly from this quest but once the quest is completed, you will be able to buy Master level conjuration spells.