Mourner gear is worn by Mourners and is obtained as part of the Mourning's Ends Part I quest. This set of clothing is required in that quest and the subsequent Mourning's Ends Part II. As a common mistake, players drop their set after Mourning's Ends Part I. It is advised to keep it stored in a Player-owned House, inside the Costume room's armour case. If you did drop full mourner gear go back to the Arandar gate. Kill the level 11 mourner, and get full gear that they drop. Beware, as he will reduce your Hitpoints and Prayer to 20.