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 legs.
The Obsidian platelegs are part of untradeable and degradeable Obsidian armour set. They require 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 their defensive bonuses (equal to torva platelegs), they provide 4% reduction in damage from creatures in and around TzHaar City. However, they are 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 platelegs degrade after 2950 hits, similar to the Barrows equipment. Once they fully degrade they cannot be reused, and new platelegs must be smithed.