What Yet Lingers is a main quest in the Jaws of Hakkon DLC for Dragon Age: Inquisition.
A mysterious island in Frostback Basin may yield clues to the region's past.
Acquisition
Upon arriving in the Frostback Basin, speak to Professor Kenric about Inquisitor Ameridan.
Walkthrough
In order to get to the island, the Inquisitor must use the boat belonging to the fisherman, Arvid Rolfsen. He will inform you that the island belongs to the Lady of the Skies and that the Avvar avoid it. He says he can only allow the use of the boat as long as the Inquisitor gets permission from Thane Svarah Sun-Hair. Head to Stone-Bear Hold and speak to Svarah Sun-Hair. She will grant you Guest status, which grants use of the boat.
Take the boat to The Lady's Rest, the island that Professor Kenric believes holds information about Inquisitor Ameridan. There, a few of the party express unease at the number of spirits that haunt the area. If you have a mage with you, they explain that the spirits are influencing minds, drawing sorrow from the living that come to the island. They will not attack you, however, so explore the island to find clues.
At the rear of the island is an alcove with a random amount of gold and loot. The moment any of it is touched five level 30 Despair demons spawn and start attacking.
Head inland and towards the destroyed shack. There, an improperly closed fade rift covers a bedroll with a skeleton, surrounded by flowers. During the conversation Solas or Cole can offer further insight to the spirit. Also in the shack is the 
Codex entry: Unfallen Shack. Open the fade rift and a spirit speaks to you. The spirit reveals itself to have the memories of Telana, Ameridan's companion and lover. She further reveals that Ameridan was sent south on Emperor Kordillus Drakon I's orders to fight an Avvar dragon, and that his mission was kept secret because of the Second Blight.
When the conversation with the spirit is done, loot the skeleton. This gives 
Heartstring Bow, Ameridan's Orders, and the ability Aegis of the Rift, which allows the Inquisitor to create a shield that deflects enemy projectiles.
In the meantime, a group of Hakkonites have landed and bar the path to the boat. They number four Hakkonite bowmen, a Hakkonite Defender with a tower shield and a Hakkonite Spellbinder mage. Dispose of them and return to Professor Kenric to inform him about what you discovered.
Rewards
- 18274 XP
Notes
As you explore The Lady's Rest you can hear Telana speaking what one can assume are her last words.
This blood... my blood? No, I can't...
Ameridan... Ameridan, why?
I can't... not without...
Vhenan... I'm... dreaming...
Sleep. I need to... I must find you...
No, no, no.