Augment Undead

Three undead creatures of your choice gain +1 to saving throws, attack and damage rolls, and are immune to being turned for the duration of the spell.
To maintain this effect after the duration, you must cast this spell again before the 24 hour period ends.
This effect does not stack.

At higher levels the bonuses increase to +2 when using a spell slot of 4th or 5th level, and +3 when using a spell slot of 6th level or higher.