The Adventurer of Revendreth achievement in World of Warcraft: Shadowlands will show you many of the rare enemies you can battle throughout one of the four zones in this new expansion. One of them is Lord Mortegore, and you need to find his scroll to summon him.
The best place to look for the Mortegore Scroll is in the Death’s End area of Revendreth. You can find Maldraxxi Defectors east of the Darkhaven flight path, located at 74.6, 63.0, which will also put you in the same general area as Lord Mortegore’s eventual spawning point. The Defectors have a very low drop rate for the scroll, so don’t be surprised to go through 20 or 30 of them before you loot the Mortegore Scroll from their bodies. You may also get sick of hearing the word “archlich” every time you kill one of them.
After you obtain the scroll, you can use it to light the four Mortegore Sigils around the area. When all four of them are glowing green, Lord Mortegore will appear. He has a high amount of health, so you might want to make this battle a group effort. After you lay Mortegore to waste, you can continue your journey toward the Adventurer of Revendreth achievement.