When the undead Scourge washed over Lordaeron, Fenris Keep was not spared. Thule Ravenclaw, an ambitious young mage, allied with the Scourge, and dominated over the local gnolls, creating the Rot Hide. He made himself master of the keep.
World of Warcraft
This section concerns content related to the original World of Warcraft.
Thule Ravenclaw commanded a large force of Rot Hide gnolls to do his bidding, which hungrily attacked anyone who neared the keep and their master. As such, they were often targeted by the Horde adventurers. They were eventually driven out of the keep by the worgen.
This section concerns content related to Cataclysm.
After the fall of the Southshore and Hillsbrad Fields towns to the Forsaken, its refugees escape towards Silverpine Forest and occupied the then empty keep as their base of operations. When agents of the Forsaken, along with Agatha, entered Fenris Keep to kill the human leaders, they were met with a surprise. The worgen leaders Darius Crowley, Ivar Bloodfang, and some of their forces were found inside trying to convince the humans to drink their blood in order to prevent the refugees from becoming part of the Forsaken. The refugees decided on the lesser of the two evils and drank Darius' blood, becoming worgen.
Battle for Azeroth
This section concerns content related to Battle for Azeroth.
By the time of the Blood War, the Forsaken forces led by Deathstalker Hayward captured Fenris Keep. Alliance champions were sent to annihilate the undead to secure Lordamere Lake, with the Bloodfang worgen eventually taking the control of the isle. This forced the Horde to create a distraction along the Misty Shore to lure the worgen out of position, allowing Horde refugees to escape Tirisfal Glades across the water.
In the RPG
Lordamere Lake contains an island called Fenris Isle. This island once held a great fortress known as Fenris Keep, but the structure has sunk into the bog and is now mostly submerged and covered with mud and scum. If you want to kill murlocs or Rothide gnolls, this is a great place to do it.