Paterne Kael
Paterne Kael asked Garon Whited:

Do you plan to end Nightlord one day or is it like Anita Blake, you will continue as new ideas comme to you? And if you can't guess it from that, I absolutely love your books, keep it coming.

Garon Whited The primary story arc is plotted for six books, so in that sense, yes, it has a definite ending.
However...
We're dealing with an immortal protagonist. There are large stretches of time involved. There may be other stories in some of the less-well-documented stretches of his long life. So it is entirely possible there will be more to read of Eric's adventures besides the six core books.
But the main story arc does have an ending, yes.

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