Thanks to datamining it has been known for a very long time now that there is a hidden Slime Serpent mount somewhere in World of Warcraft's newest Shadowlands expansion. The obvious place to check was the Plaguefall dungeon given that you can see said serpent swimming through the ooze-filled water, yet despite all of this knowledge and a considerable amount of effort on the part of the playerbase, no one could actually figure out how to get it.

Then, as is often the case with these things, the answer was discovered by pure chance. While going through Plaguefall as a part of his series to solo every single Mythic dungeon, Durendil found the mystical Slime Serpent just kind of chilling next to the third boss after he cleared the dungeon!

What followed next was a massive rush of people trying to figure out what exactly the requirements for the Slime Serpent even are. Well, the answer is thankfully about as simple as it gets! All you need to do is defeat the final two bosses in Heroic or Mythic Plaguefall, after which the Slime Serpent will spawn near the third boss. Once you've done all of that simply click on the Slime Serpent and you'll be granted a shiny new mount!

The only catch is that soloing the dungeon might be incredibly difficult for you depending on your spec and item level. I did it on a well geared Demon Hunter tank and so the whole process was quite relaxing, but I'd imagine squishy classes like Rogues might struggle considerably. So unless you have a tank alt that can blaze through the dungeon, you might need to look up a guide and see if it can be done with through sort of clever trickery.

Good luck with Plaguefall, and here's hoping you'll give the Slime Serpent a loving home!