So I’ve gone through Exploring Azeroth: Northrend which I got for my birthday awhile ago. There were a couple of mentions of San’layn, but it was more of a recap of events in WotLK as opposed to anything current or of special interest. I’ll quote the book where it was mentioned.
As a reminder, the book was written from the point-of-view of Muradin, Brann, and Magni, who have strong ties to the Alliance. So even if some San’layn were involved in the book, it likely wouldn’t have been too much of an event I’d think.
Book quoting as follows:
Utgarde Keep - P. 20
“The Alliance with the Scourge meant that the Lich King sent an ambassador, Prince Keleseth, to reside at Utgarde. Keleseth was a San’layn, which are a sect of darkfallen elves, all of them undead and obsessed with blood magic. He oversaw Scourge operations in Howling Fjord and made himself a general pain in the rear around the region, working with Ingvar the Plunderer to organize raids. Adventurers eventually killed him, but like all the most annoying Scourge, he didn’t stay dead. His dagger is just what you’d expect from a mage: not much of an edge to it, but it packs an arcane punch.”
Temple City of En’kilah - P. 108
“The necropolis Naxxanar still floats over En’kilah. but there doesn’t seem to be much going on there anymore. While the Lich King was active, Naxxanar was the command post for the Scourge in the Borean Tundra, run by Prince Valanar, the overlord at the time. It was here the brave death knight Thassarian faced off with Prince Valanar and discovered that he’d been trying to destroy the Valiance Expedition from within by masquerading as Counselor Talbot. Valanar had Thassarian’s sister, Leryssa, and General Arlos under mind control and was about to hand them over to the Lich King. Thassarian defeated Valanar to save the both of them.”
A couple of notes to make:
-Did not see anything mentioned about Prince Taldaram or Queen Lana’thel.
-The Violet Hold in Dalaran has a small section, but nothing specific about its prisoners like Blood Princess Thal’ena.
-Nothing really said about San’layn in general I saw aside from the two quotes above. Just the two instances in the past as opposed to anything current.
-The Scourge in general are said to be aimless and not really controlled by anything. More on this in a bit.
And some ramblings I’ll make with some of the information:
-There may or may not be any San’layn on Northrend. If there are, they’re hiding out somewhere. Otherwise, they may have left and gone elsewhere for…reasons. Maybe some did actually get free will and didn’t want to be near the Scourge? Wouldn’t blame them, given that they fought the Scourge in life.
-Unlike the Scourge, the curse the San’layn have may not be affected by the whole Helm of Dominance being destroyed and Crown of Wills being made event, which could explain why Vorath acted the way he did. Or it could be that San’layn did get free will, albeit still cursed with vampirism and have to deal with it and some either go mad from it or some control it and become power hungry or something. This could work well into their storyline in overcoming their curse and controlling it.
Several more questions pop up regarding San’layn than answers. I still think San’layn could be fleshed out to be much more, with plenty to work with them joining the Horde and becoming playable. Hopefully, Blizzard would see that potential someday as well.
-Still dreaming of playable Horde San’layn as a full race! 
