The stalker gland is an item that could be obtained from 8 January until 22 January 2018 from the Guardian of the Vault, by players who purchased the 2018 Premier Club Gold package. It was released along with the addition of seekers and soulgazers as Slayer monsters in the Stalker dungeon.
The gland can be exchanged with Kuradal or Morvran to ensure stalker creatures as the player's next Slayer assignment. The assignment will automatically be given when asking for a new assignment from the master with the gland in the inventory.
Players who have not completed Dishonour among Thieves could obtain this item, but will not be able to get the task until the quest is completed.