"Until the Rebellion, thrulls served their masters faithfully—even at the cost of their own lives."
—*Sarpadian Empires, vol. II*

Thrull Retainer

Enchant creature
Enchanted creature gets +1/+1.
Sacrifice this Aura: Regenerate enchanted creature.

Masters Edition

Enchant creature
Enchanted creature gets +1/+1.
Sacrifice Thrull Retainer: Regenerate enchanted creature.

Fifth Edition

Enchanted creature gets +1/+1.
Sacrifice Thrull Retainer: Regenerate enchanted creature.

"Until the Rebellion, Thrulls served their masters faithfully—even at the cost of their own lives."
—*Sarpadian Empires, vol. II*

Fallen Empires

Target creature gets +1/+1.
Sacrifice Thrull Retainer to regenerate the creature it enchants.

This leaves the battlefield when you activate its activated ability, but the enchanted creature won't be regenerated until the ability resolves. In the intervening time, the creature will no longer have the bonus that this had been giving it. This may cause the creature to be destroyed before the regeneration shield is created.