The quest The Brink of Extinction must be completed. And higher combat stats are recommended.It may take 1.5 hours or longer to get the helm.
The Obsidian ranger helm is part of the untradeable and degradeable Obsidian armour set. It requires 80 Defence to wear, and can be made by smithing two obsidian bars, requiring 84 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 the pernix cowl), it provides 3% 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, it will increase your Ranged accuracy by 10%, very similar to the Void Knight equipment and void ranger helm.
The Obsidian ranger helm degrades after 2850 hits, similar to the Barrows equipment. Once it fully degrades it cannot be reused, and a new helmet must be smithed.