Some information in no particular order about San’layn:
BfA, Crimson Squall - reaction => Alliance (Hostile) / Horde (Friendly)
Some nice quotes of blood with San’layn Neophytes:
- I’ll drain the flesh from your bones!
- Let me hear you scream.
- You are but fuel for the san’layn!
- One… last… drop…
- The Blood Prince… will… drain you dry…
An Unnatural Crew (Alliance quest - BfA):
"Blood Prince Dreven is here, and the situation’s even worse than we thought.
That monster’s training other san’layn. Every neophyte you see on this ship is another future Blood Prince Dreven.
His vile practices cannot be allowed to spread within the Horde. We’re going to tear the san’layn out, right now, root and all. "
=> That monster’s training other san’layn. Every neophyte you see on this ship is another future Blood Prince Dreven.
Something about what are San’layn:
Caladis Brightspear says: I knew this was a dragonforged blade when I first laid eyes on it.
Caladis Brightspear says: But can it be? Is this really –Lana’thel appears out of nowhere walking up to Quel’Delar from the southwest. The four sentinels and war mages rush to the sword.
Silver Covenant Sentinel says: You are not welcome here, minion of the Lich King!
Lana’thel says: Quel’Delar.Lana’thel freezes the war mages and sentinels surrounding the sword.
Lana’thel says: Quiet, fools. If I had wished to kill you, you’d already be dead.
While Lana’thel speaks, images of what she talks about appear just north of the blade.
Lana’thel says: As Quel’Serrar was forged by the dragons and given to the kaldorei, its twin, Quel’Delar was given to my people.
Lana’thel says: The king bestowed the blade upon my friend, Thalorien Dawnseeker.
Lana’thel says: But even Thalorien’s skill and Quel’Delar’s magic could not save Silvermoon from the might of the Scourge.
Lana’thel says: Thalorien fell before the gates of the Sunwell, buying time for others to escape.
Lana’thel walks north and kneels down to inspect the image of Thalorien.
Lana’thel says: After the battle, I recovered the sword from the field. I bore it to Northrend in the service of my prince, seeking to avenge our people’s defeat.
Lana’thel says: Arthas shattered our forces and took the most powerful of us into his service as the San’layn.
Lana’thel says: My memories of Thalorien went cold and so did his blade.
Lana’thel says: It was I who brought Quel’Delar here, to return it to its makers. In breaking the weapon, so did I break its power.
Lana’thel says: Quel’Delar will never serve another!The freeze fades and Lana’thel appears to cast some spell on Quel’Delar, disappearing once again.
Caladis Brightspear says: I vow that the Silver Covenant will see Quel’Delar restored.
(“Return To Caladis Brightspear” => with Arcanist Tybalin for the Alliance side / “Return To Myralion Sunblaze” => with Magister Hathorel for the Horde side - WotLK)
Something else about what are San’layn:
…
Following the destruction of their homeland, the determined blood elves began to regroup and rebuild. It was during this time that the battle-hardened warrior Lana’thel stumbled upon Quel’Delar, and soon afterward the greatest opportunity to use the blade presented itself: Prince Kael’thas Sunstrider raised an army and combined forces with Illidan Stormrage, who led them north to Icecrown on a mission to destroy the Lich King. Lana’thel readily followed her prince with one thought in mind – revenge against the Scourge – but only tragedy awaited her.In the icy hills of Northrend, Arthas ultimately defeated Illidan and Kael’thas, and the expedition’s few survivors were scattered across the frozen wastes. One by one the Lich King sought out those who had challenged him, and Lana’thel found herself cornered. Using Quel’Delar, she struck at him with all her might… but Frostmourne’s evil powers overwhelmed her. Through his blade, the Lich King chose Lana’thel to serve him in undeath as queen of the San’layn, a group responsible for overseeing the Scourge’s operations across Azeroth.
…
(“Underdev/Quel’Delar: The Sister Blade” - an official article by Blizzard Entertainment from Oct 30, 2009 but it did not survive the overhaul of the World of Warcraft Official Website)
They seem to drink blood and they seem to be “happy” with that:
They’re… bleeding the prisoners… putting the blood… in those globes.
<Grumm’s eyes close momentarily and he leans his head back.>
The blood… taken up to necropolis… to feed blood prince…
P-promise… you’ll… destroy blood… globes… bring word… to Junek.
(“Shatter the Orbs!” => Horde quest in the Borean Tundra - WotLK)
“- One… last… drop…” - San’layn Neophyte
“Dinner… is served.” - Prince Valanar
“Just a taste…” - Vampiric Bite, Blood-Queen Lana’thel
Lana’thel was one of the undead forces reanimated by Mawsworn kyrian in Aldur’thar: The Desolation Gate in Icecrown. (Shadowlands prepatch)
San’layn can be Warriors (Darkfallen Commander), Rogues (Darkfallen Tactician), Priests (High Priest Andorath), Mages (Darkfallen Archmage), Warlocks (Darkfallen Noble)Death Knights (Darkfallen Deathblade) and Blood Knights.
Even with the fall of the Scourge and of their Blood-Queen Lana’thel, San’layn seem to be able to have a kind of hierarchy:
- Blood-Queen/Blood-King (none for now)
- Blood Prince (Blood Prince Dreven)
- Marquess / Marquis (Blood Marquess / Blood Marquis)
- Neophyte (San’layn Neophyte)
Bonus 
