I feel as though the Green Knight and the Dark Prince may have shared an enemy at a time. Perhaps something can be arranged?
Definitely, they certainly have a similar obsession with killing all manner of undead.
It might provide some interesting answers as well as to how the two can or might get along, considering one is an undead vampire and the other a knight tasked with slaying unnatural creatures.
Randomly brainstorming, perhaps, if they've fought a common foe there is some level of respect (something like, technically the Green Knight would simply try to slay the ancient vampire, but given evidence of his "goodness" perhaps he's willing to let it slide).
Feel free to send me a PM if you have some ideas, just so we don't kill the thread with back and forth ideas.
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Edit: I extend the same offer to anyone else, the Green Knight's travels have assuredly seem him encounter many a hero over the ages.