Necromancy Master (5e Feat)
From D&D Wiki
Prerequisites: The ability to create Undead, 8th level or higher
You have mastered the art of bending Undead to your will.
- You can create undead using the remains of dead creatures.
- When you cast a spell that creates or asserts your control over undead, such as animate dead, you can create or assert control over one additional undead.
- As a 10 minute ritual, you may expand three spell slots of 3rd level or higher and two spell slots of 4th level or higher on one undead currently under your control to make them serve you permanently. The undead must remain within 30 feet of you during the entire ritual. If the undead isn't currently under your control, the ritual takes 30 minutes and an additional 5th level spell slot. You may telepathically communicate with any undead you control in this way as long as you're on the same plane.