Cave krakens are Slayer only monsters, meaning they can't be attacked when not on a Cave kraken task. You'll need to have 86 Slayer in order to get the task.They can only be ranged or maged as they're in the water. It's advised to use "Protection of magic" as the Cave krakens do only magic attacks and they do minimal damage though the prayer. They can drop the Trident of the seas and also the Cave kraken pet making it a valuable slayer task.