Breath of Osiris

This spell creates an undead servant. The soul, body and death of that creature are needed to turn it into a servant, and only creatures within half of the player's level. To create the summon, the player has to roll 1d6, and has three chances to successfully summon the creature. Every day that the creature  remains dead is -1 to each of the rolls, and each level it is above the allowed level (half of the player's level) is another -1 to every roll. The creature also must be killed by the summoner's hand, or summon.

On each of your turns, you can use a bonus action to mentally command any creature you summoned with this spell (if you control multiple creatures, you can command any or all of them at the same time, issuing the same command to each one). You decide what action the creature will take and where it will move during its next turn, or you can issue a general command, such as to guard a particular chamber or corridor. If you issue no commands, the creature only defends itself against hostile creatures. Once given an order, the creature continues to follow it until its task is complete or until that creature is returned to nothingness.

The creature is under your control for 24 hours, after which it stops obeying any command you've given it and returns to nothingness. At the first level, your summoning slots are limited to 2 saved creatures and 3 summons (you can summon 3 at a time, but only keep 2).

Initially summoning your summons, not from their original body but after being saved, takes one spell slot to summon all of them at the same time. If you wish to summon them separately, additional spell slots are needed. If your summons are destroyed after being saved, you are capable of reviving them by expending on spell slot.

At Higher Levels:

3rd Level. The summoner is now capable of creating skaabs, a type of undead. To achieve this, the summoner must have at least 3 dead bodies, race does not matter. They then must succeed in a Arcana DC 13 check. Every time they fail, a body is trashed, thus requiring another for the ritual. Skaabs do not need a summon slot to be kept, as long as they are made by the summoner.

4th Level. When using this spell at 4th level or higher, an additional summon slot is added. As well as that, each spell level also adds another spell slot.

5th Level. When animating summons at 5th level and above, you no longer have -1 on your rolls. You also gain spiritual sight, allowing you to see through your summon's eyes.

7th Level. You gain an ability called spiritual mark, allowing you to summon creatures without expending a spell slot. Spiritual mark also enables you to transport yourself to your summon's position, while your summon appears in yours. This recharges every short rest.

10th Level. When you create a skaab, you are now capable of combining it with different summons or creatures, increasing it's strength.

15th Level. Limits on what creatures you can create into summons based on your level are now eliminated. As long as a creature is dead, you can take it's soul and make it a summon.

20th Level. All skaabs made are instantly perfect, able to pass for human. You are able to summon unlimited creatures, reviving them no longer takes spell slots, you can also save unlimited creatures. Spiritual mark has a one round cool down, and skaab creations are now available with only one body (as long as it fits the creature's size scale) and can be created just from seeing a creature.