Skyrim:Dead Thrall

Dead Thrall is a master level Conjuration spell which raises a dead body permanently to fight for you.

Other reanimate spells are temporary, and after the duration of the spell, the undead minion disintegrates into a pile of ashes, forcing you to look elsewhere for a new minion. However, Dead Thrall minions are permanent and can be raised again as many times as needed if they die.

Obtaining the Spell
Level up your Conjuration to 90.
Complete the Conjuration Ritual Spell quest for Phinis Gestor to be able to purchase the Dead Thrall spell tome.

Obtaining a Thrall
Any humanoid level 40 or below may be reanimated to become a permanent minion. However, there are some considerations which might guide your decision in choosing a minion since the thrall is permanent and can be raised again if killed during combat. Note that some leveled NPCs may become unusable once you reach level 40, as they keep leveling while risen as a thrall, which may make the Dead Thrall spell ineffective if the thrall happens to die in combat and you attempt to raise them again. For this reason, you may select a thrall that has a level cap at or below level 40.

Types of Thralls
Unique NPCs and Followers
Many unique NPCs will retain their unique abilities when reanimated, such as Orchendor's teleportation or Ulfric Stormcloak's shouts.
Some unique NPCs are leveled so care should be taken when enthralling them, as they might cross the level cap.
Generic NPCs (bandits, vampires, etc.)
Dremora

Permanent Thrall Corpses
Only a few NPC corpses will not de-spawn if you leave the cell and return later. This can be beneficial for players who want to collect and store the bodies of useful thralls for later use. This applies so long as the NPC was initially killed via conventional means and was not decapitated, dismembered, or turned to dust; via skill perks or via lesser reanimation methods. A list of them may be found here.

Non-respawning named NPCs can be turned into permanent corpses by placing a permanent item onto them. As long as the item is in their inventory, they will not disappear. Respawning NPCs and creatures cannot be made into permanent corpses this way.

Notes
Dead Thralls will equip any weapons and shields that you place on their corpse prior to reanimation (requires a loading screen or engaging an enemy to initiate equipping). Just like other followers, they will prefer weapons according to the quality of the material rather than the smithing level of improvement. They will also use staves, including those which summon or reanimate, and will often prefer them over melee weapons.
Dead Thralls will only equip the exact armor that they spawned in, so a corpse wearing a full set of elven or glass armor may be more durable in combat than a bandit in fur armor. You may take the armor from the corpse in order to improve it, enchant it, and even rename it and then replace it onto the thrall, but it must be the very armor the thrall spawned in for them to re-equip it, not just a similar item with the same name. As a result, corpses which had more equipment slots filled in life make optimum thralls since more armor pieces can be improved and enchanted to enhance the thrall's survivability. Unnamed NPC thralls spawn their armor and weapons based on different loot tables. Taking all the equipment on a thrall corpse and then using the dead thrall spell will cause the thrall to respawn its armor from said loot table, upon entering any loading zone. Multiple attempts at this is a way to acquire multiple copies of a certain type of armor, as well as a way to try to acquire slight variations of armor type or design from that NPC's loot table.
Forsworn and Dawnguard thralls may equip any Forsworn or Dawnguard armor given to them, respectively, due to the sharing of loot tables.
Nightlord and Nightmaster vampires from Dawnguard will likewise equip any vampire armor and boots.
Equipping your thrall with any piece of enchanted gear negates the bonus from the perk Dark Souls, which normally increases your thrall's health by 100.
Thralls can only take advantage of the same enchantments as standard followers. Using a Fortify Unarmed Damage enchantment on bracers, it is possible to make Khajiit Bandits or Bandit Outlaws fight unarmed in preference to using a weapon - a rarity for a humanoid NPCs.
Some thralls do not groan and may either be silent or retain their original vocal clips as when they were alive. Typically, these are named NPCs associated with a particular quest that would not exist outside of that mission (such as the dead mages in Nchuand-Zel). Notably, Dremora and many unnamed vampires retain their standard vocal clips as thralls.
Dead Thralls behave the same as other reanimated NPCs.
When you fast travel to a major city, your thrall is left behind at the city stables to be picked up later.
Like all reanimate effects, the level cap is boosted by 25% with the Necromage perk. This allows you to reanimate NPCs up to and including level 50, but only if the NPC was undead prior to being killed and reanimated, such as a vampire. 
This can be further increased to level 62 by having the Bone Wolf pet or Bloodworm Helm, allowing you to permanently reanimate Nightlord Vampires, or to level 78 by having both, allowing you to permanently reanimate any generic vampire and any named vampire with a maximum level, Arch-Curate Vyrthur being the highest with a max of 75.
The Dead Thrall spell is also subject to the Twin Souls perk, effectively allowing you to have two permanent thralls at any given time.
You can summon a third dead thrall if you use the Staff Of Worms while wearing armor enchanted to increase the number of undead minions by one.
The Wabbajack will exclusively deal elemental damage to Dead Thralls, and cannot be used to transmogrify or instantly kill them.

Bugs
 
 
 
 
  
This bug is fixed after installing Dawnguard.
 
The thrall that has gone missing may be back where you last had it, or where the NPC originally spawned. Finding and killing the thrall will remove the effect.
 This bug can be fixed by entering in the console  .
Transforming into a Werewolf or Vampire Lord can fix this issue.
 

See Also
Reanimate Spells
Reanimate Effect