I can tell you what I found in the archives, which is basic stuff and also says that each Dark Wizard who binds their followers creates their own Dark Path and they will be unique. It's all specific about how to break it, and it's not very optimistic, but it has lots of detail on things not to do. Quite graphic
So you know, none of this may apply in this particular case.
There are two general classes, once is a living spell, which is an evocative name for a spell that has been programmed to evade detection and to respond to its master. The other is a type of feudal binding. Basic contract is promise of protection for promise of service. In general magical bindings, neither of those is common any more, outside of dark wizardry, and that's not that common.
Both types are effectively a mark of ownership by or allegiance to a Dark Wizard. They can't be seen by others unless the Wizard or the Marked wants them to be seen. They're very difficult to detect, especially if the Marked isn't cooperating. This makes them difficult to use in prosecutions, because they're hard to prove or disprove.
They're often tied to a power to compel someone, either to attend the Dark Wizard or otherwise to do something. They have been known to denote status amongst the followers, and allow one minion to give orders to the unmarked, or two marked minions to recognize each other. That's pretty suspect, though. You'd have to risk exposure to do that.
They're said to be difficult to remove, but they will fade if the tie is broken. Historically, it doesn't seem to prevent murdering your master and taking his place, but it requires something like that to break it. There's an account of a witch with a mark on her hand. She cut off her hand and re-grew it, and it was still marked.
Have you seen one? I don't know of any proof that He Who Shall Not used them, but it's quite likely, and rumors abound.