After you saved Amyro (The Imprisoned Elf), you are asked to inform the elves on mainland that Saheila's fate.
Travel to Elf Encampment at Reaper's Coast (south to the sawmill), and inform the elves there.
The Elves at the camp are burying one of their fallen (Burial Rites), if your character is an elf, s/he can complete the quest with ease. If not, you need to pass persuasion check. Or, Use shapeshift mask to try again if all your characters failed.)