What the achievement asks
Restore the Avatar of Sethraliss in the Temple of Sethraliss on Heroic difficulty or higher. Ten points, both factions.
Note the verb: this is the one dungeon of the eight that does not end with a kill. The last encounter restores the snake loa rather than destroying her, which is why the achievement reads differently from its seven siblings.
The four encounters
| Order | Encounter |
|---|---|
| 1 | Adderis and Aspix, a paired fight |
| 2 | Merektha |
| 3 | Galvazzt |
| 4 | Avatar of Sethraliss |
The temple is in Vol'dun, built around the remains of the loa who died stopping Mythrax, the same Mythrax you fight much later in Uldir.
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.