Nazjatar Blood Serpent
The Nazjatar Blood Serpent is a secret mount added in Battle for Azeroth. This serpent is a red and grey recolor of the Abyss Worm and Riddler's Mind-Worm mounts from Legion.
First you'll need to collect 20 https://www.wowhead.com/item=161344/abyssal-fragment. They can drop from any creature in Kul Tiras and Zandalar, but they are not soulbound, which means you can also find them on the Auction House.
Once you collect all the fragments, you will need to combine them in front of the Altar of the Abyss, in a cave located behind a waterfall in Stormsong Valley, to create the https://www.wowhead.com/item=161345/abhorrent-essence-of-the-abyss.
/way Stormsong Valley 73.50, 23.61 Altar of the Abyss
Adherent of the Abyss
Next you will have to summon the https://www.wowhead.com/npc=140474/adherent-of-the-abyss by using the https://www.wowhead.com/item=161345/abhorrent-essence-of-the-abyss in front of The Abyssal Flame, which is also located in Stormsong Valley at Warfang Hold.
The https://www.wowhead.com/npc=140474/adherent-of-the-abyss may be hard to solo for undergeared characters, so make sure to bring a few friends to help out.
/way Stormsong Valley 46.47 36.03 The Abyssal Flame (cave entrance)
/way Stormsong Valley 45.08 36.76 The Abyssal Flame
Once you kill the Adherent, it will drop the https://www.wowhead.com/item=161479 for the summoner only, and https://www.wowhead.com/item=163929/aether-of-the-abyss for everyone in the party. This item can only be obtained once.
The https://www.wowhead.com/item=163929/aether-of-the-abyss is part of a still unsolved secret. If you use it, it will cast a purple explosion around you and knock you down. No further info has been discovered as to what using the item accomplishes.
The https://www.wowhead.com/npc=140474/adherent-of-the-abyss will shout several sentences in [Shath'Yar] language:
Adherent of the Abyss says: [Shath'Yar] ZUQ THYZZ Zuq ag'xig qwor ni ak'agthshi oou kla'kar zuq nuq n'zoth shfk
Adherent of the Abyss says: [Shath'Yar] I zz AGTH I plahf zuq yrr nuq ywaq on'ma lwhuk zaix za sk'woth'gl uhnish
Adherent of the Abyss says: [Shath'Yar] Shuul og i uull ryiu hnakf wgah iiyoq shg'fhn yrr zuq
These have been translated using a https://www.wowhead.com/item=162011/violet-truffle:
Adherent of the Abyss says: [Shath'Yar] YOU FOOLS! You cannot hope to comprehend the power you now trifle with!
Adherent of the Abyss says: [Shath'Yar] I am FREE! I thank you... and now your souls shall sate my fathomless hunger.
Adherent of the Abyss says: [Shath'Yar] There is a fate much worse than death waiting for you...