What the achievement asks
Defeat the Unbound Abomination in Underrot on Heroic difficulty or higher. Ten points, both factions.
Underrot sits beneath the swamps of Nazmir, and it is the shortest of the eight Battle for Azeroth dungeons: four encounters and out.
The four encounters
| Order | Encounter |
|---|---|
| 1 | Elder Leaxa |
| 2 | Cragmaw the Infested |
| 3 | Sporecaller Zancha |
| 4 | Unbound Abomination |
With only four bosses and no optional detours, this is the quickest of the eight to clear and a sensible one to start with.
Heroic or higher, but on one character
Blizzard's wording is "on Heroic difficulty or higher", so a Mythic or Mythic Plus run earns it just as well. There is no need to queue for Heroic specifically.
The eight Heroic dungeon achievements are not account-wide. Four on your main and four on an alt leaves you with nothing. They all have to be earned by the same character.
Once one character holds all eight, the Battle for Azeroth Dungeon Hero achievement they feed into is account-wide, so the grind only has to happen once.