The keris is a dagger obtained after defeating the giant scarab at the end of the Contact! quest. It deals 33% bonus damage against all Kalphites and Scabarites, and has a 1/51 chance of puncturing a hole in their exoskeleton, dealing triple damage. When the special effect occurs, the chatbox will display the message You slip your dagger through a chink in the creature's chitin, landing a vicious blow.
Keris requires 50 Attack to wield. Like most daggers, it can be poisoned. It is lost on death if it is not protected, and can be retrieved from Osman for a fee of 20,000 coins. Its stab attack is identical to that of a dragon dagger, although its Strength bonus is 10 lower. It has the second-highest score of the three daggers that provide prayer bonuses, with a +2 compared to the white dagger's +1 and Wolfbane's +5, but is physically stronger than either of those. It is needed to kill a Locust rider or Scarab mage for a Hard Desert Diary task.
For an exhaustive list of all known sources for this item, see here