Death Puppeteering

You reanimate and enslave a Creature with a CR of up to half your level, rounded down. The Gem used during the spell becomes the Creatures Shackle. When the Gem is removed, either by the creature suffering a critical hit, or being removed with a Sleight of Hand check with a DC of the creatures passive perception plus its dex mod, it crumbles to dust and the creatures strength begins to fade and it crumbles after 1d6 turns, all of its rolls being reduced by the amount of turns that have passed since the crystals removal. The Spellcaster can use his reaction to spend a total amount of Spell slots equal to the creatures CR to prolong its life by the casters spellcasting modifier (Example: CR4 Creature has crystal removed, 16 Int Wizard can use a third level and first level spellslot to keep it alive for 3 more turns). Once the Catalyst is removed, it regains its free will, and depending on how it was treated by its master, will continue to follow his commands or turn on him. If spellslots were used control is maintained for the additional turns.

If the Creature enters an anti magic field it becomes incapacitated, if it remains there for more than an hour it crumbles.

The Creature cant be charmed or frightened by any means.

If the creature dies, it can be reanimated as long as the crystal isnt damaged with 1 hp by spending an appropiate spell slot.

You can only control an amount of undead this way up to equal to your Spellcasting modifier. 

In order to maintain the undead a spell slot one level below the one used in casting is lost until control over the undead is lost.

The Creature loses all Bonus Actions, Spells, Reactions, and Legendary Actions. Its Statblock remains the same with a few changes depending on the creatures form:
Zombie: Gains Undead Fortitude Feature, an additional hit die, Poison Immunity and Piercing Resistance, Slashing Weakness, Fire Weakness, Radiant Weakness and loses 10 movement.
Skeleton: Piercing attacks against it are at a disadvantage, Poison Immunity, Slashing Resistance, Bludgeoning Weakness, Radiant Weakness, loses flight if it relied on Wings in Life.

You can choose to expend a hit die during a short rest to heal all undead created this spell by your spellcasting modifier + Proficiency Bonus.

When using a 5th level spellslot, the possible size category of the creature increases to large, the creature gains access to a pseudodragons limited telepathy and it can use its Bonus Actions, if it had any in life. If it has the spellcasting Feature, it gets 2 necromancy spells of a level no higher than its former spells, with the sole limitation of your DMs approval.
When using a 7th level spellslot, the possible size category increases to huge, the creature gains access to pack tactics and it can use its Reactions if it had any in life.  If it has the spellcasting Feature, it gets 4 necromancy spells of a level no higher than its former spells, with the sole limitation of your DMs approval.
When using a 9th level spellslot, all sizes are supported, the creature gains proficiency in 1 skill and expertise in another and it can use its Legendary Actions, if it had any in life.  If it has the spellcasting Feature, it gets 6 necromancy spells of a level no higher than its former spells, with the sole limitation of your DMs approval.