The quest The Brink of Extinction must be completed. And higher combat stats are recommended.It may take 2 hours or longer to get the skirt.
The Obsidian plateskirt is part of untradeable and degradeable Obsidian armour set. It requires 80 Defence to wear, and can be made by smithing three obsidian bars, requiring 88 Smithing. Obsidian bars are made by smelting obsidian shards, which in turn are dropped by monsters in the Fight Cauldron.
In addition to its defensive bonuses (equal to torva platelegs), it provides 4% reduction in damage from creatures in and around TzHaar City. However, it is only useful when fighting TzHaar and TokHaar -- outside that area, and in PvP, the obsidian armour will triple damage towards you.
When used with the rest of the obsidian armour and either an melee, ranged or magic helmet, it will increase your accuracy by 10%, very similar to the Void Knight equipment.
The Obsidian plateskirt degrades after 2950 hits, similar to the Barrows equipment. Once it fully degrades it cannot be reused, and a new plateskirt must be smithed.