Gift from Sliske is an achievement that requires the player to find Sliske at the Empyrean Citadel to get the permanent effect of the ring of visibility, and bring the Sixth-Age circuit to the Guthixian High Druid to add the teleport to the World Gate to the ring after completion of Fate of the Gods.
Regardless of whether Sliske's Endgame is completed or not, Sliske can be found by entering the throne room in the Empyrean Citadel, walking to the middle of it and then walking away. Sliske will then appear on the stage and after a short dialogue, the ability to always see into the Shadow Realm is given to the player.