The regeneration effect of the Lifeblood Spear works on thralls. Stick it into their hands and their health points shoot up insanely fast. They can recover thousands of damage in under a minute.
Just like players, they will not heal if they take damage, so having them use the spear as their primary weapon will not necessarily make them immortal - during combat, that is. Since thralls usually automatically put their weapon away after a battle, and take it back out to engage the next enemy, they will automatically start the regenerative effect again just before every fight. And since the Spear heals thralls so fast, they will likely recover hundreds of health before they lose the effect again to damage, even if the enemy hits them within the first seconds of the conflict.
This means a thrall with a Lifeblood Spear will almost certainly heal faster than enemies can hurt them unless they are involved in a very long fight against a very strong enemy, or get into new fights too quickly to ever put away their weapon in between.
Downsides include the difficulty in acquiring the Spear in the first place, the fact you can’t equip it on pets, and if you don’t want your thrall to use a polearm, you’ll have to heal them by manually handing it to them outside of combat.