You can cast this spell only at night and only with a powerful source of magic nearby. The ritual to cast the spell requires complex and complicated ceremonies some of which require knowledge that might not be readily available. Possibly requiring the help of an entity which has the knowledge, or seeking ancient forbidden knowledge. The ritual takes place over a week and requires a steady stream of sacrifices to build the Death Knight's body, on the final day, the still beating heart must be incorporated into the Death Knight's body, and it will finally create the Death Knight (The GM has the statistics for this monster)
As a bonus action on each of your turns, you can mentally command the Death Knight animated with this spell if the creature is within 120 feet of you (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 Death Knight 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, theDeath Knight only defends itself against hostile creatures. Once given an order, the Death Knight continues to follow it until its task is complete.
The Death Knight is under your control until dispelled or until it dies.
* - ((skull of a gladiator(or equivalent), skeleton of a famous hero (or equivalent) still beating heart and a diamond worth 30,000gp that must be ground to a fine dust mixed with the skeleton and skull) )