Obsidian platelegs are a piece of the obsidian armour set and can be purchased in TzHaar-Hur-Zal’s Equipment Store for 100,500 Tokkul (87,100 with Karamja gloves). It requires level 60 in Defence to wear.
When used in conjunction with the obsidian helmet and the platebody, all obsidian weaponry will be given a 10% boost in melee accuracy and melee strength. The damage bonus does stack with the Berserker necklace.
Obsidian platelegs are one of the only two leg armours that give a strength bonus, the other being the Bandos tassets. Compared to them, obsidian platelegs have far worse defensive stats (just slightly less than rune platelegs), lack of prayer bonus, and have 1 less strength. However, obsidian armour is significantly easier to obtain.