In Embers of Neltharion, Blizzard is implementing cross-faction guilds, and Hall of Fame achievements have been slightly adjusted to accommodate the new feature.
For Aberrus, the Hall of Fame achievement is Hall of Fame: Scalecommander Sarkareth.
Hall of Fame: Scalecommander Sarkareth – Among the first two hundred guilds worldwide to defeat Scalecommander Sarkareth on Mythic difficulty.
Title Reward: Famed Slayer of Sarkareth
The change makes perfect sense and instead of being among the first 100 Alliance or Horde guilds worldwide to defeat a raid boss on Mythic Difficulty, the faction requirement has been removed and the achievement can be earned by the first 200 guilds worldwide to defeat Sarkareth on Mythic.