You use a Mithral Dagger to server the shadow of one creature creating a power servant that obeys the creature's command. The shadow is a creature, partially real and formed from shadows, and it can take actions and otherwise be affected as a normal creature. It appears to be the same as the original, but it has half the creature's hit point maximum and can not wear equipment. Upon creation of the shadow, the creature takes 4d10 necrotic damage and gains two exhaustion levels. Otherwise, the shadow uses all the statistics of the creature it duplicates, except that it is an undead.
The shadow is friendly to you and creatures you designate. It obeys your spoken commands or telepathic commands within 150 feet, moving and action in accordance with your wishes and action on your turn in combat. The shadow lacks the ability to learn or become more powerful, so it never increase its level or other abilities.
If the shadow is damaged, it can regain lost hit points and expended spell slots only when it returns to the creature for the period of a short or long rest. If the shadow drops to 0 hit points it admittedly returns to the creature, who can call upon the shadow after the completion of a long rest.
If the creature drops to 0 hit points it gains full control of the shadow. If the shadow returns to the creature before it makes all of it's death saving throws the creature instantly regains consciousness and regains hit points equal to the amount the shadow had.
If the shadow is stabbed with the Mithral Dagger used to create it, the shadow is destroyed and the creature takes 8d8 + 40 necrotic damage. If the creature falls below 0 hit points it doesn't make death saving throws and instantly dies. Otherwise the creature regains it shadow but can not be call upon the shadow again.
* - (a diamond worth at least 500 gp, which the spell consumes, and Mithral Dagger worth at least 1,500 gp )