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. 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.
It is lost on death if it is not protected, and can be retrieved from Osman for a fee of 20,000. They can also reclaim it from Perdu for 25,000 should they lose it.
A player wielding Keris.
For an exhaustive list of all known sources for this item, see here