Skeleton Key is malware used to inject false credentials into domain controllers with the intent of creating a backdoor password.  Functionality similar to Skeleton Key is included as a module in Mimikatz.
|Enterprise||T1556||.001||Modify Authentication Process: Domain Controller Authentication||
Skeleton Key is used to patch an enterprise domain controller authentication process with a backdoor password. It allows adversaries to bypass the standard authentication system to use a defined password for all accounts authenticating to that domain controller.