Mourners are a collection of elves and humans who have joined the Prifddinas Death Guard. They are located all across West Ardougne, and some are located at the gate of Arandar (entrance to Tirannwn). Some of the Mourners drop the mourner outfit, items used in Mourning's Ends Part I and Part II. As a side note, use caution when fighting the level 11 mourners as they will drain all your combat stats (including Hitpoints) to 20 when attacked.
The level 108 Mourners located in the basement of the Mourner HQare considered as Elf warriors, so their stats and drop table is the same as an Elf warrior, with the exception of ensouled heads and seeds, and they will also count towards an elf slayer task. After Mourning's Ends Part II, the basement can be accessed easily by selecting the Dark beasts teleport on a slayer ring, and running through the door to the east.