To get this pretty amazing earlygame Grappling Hook-like weapon you'll have to murder a boatload of Cave Bats. They have a 0.4% chance of actually dropping this which seems stupid to me. It definitely needs to be upped as the majority of players won't even see this weapon once during a playthrough. You can also farm it using the bat statue but by that point you'll have something better.
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The Chain Knife is a rare drop from cave bats. It deals 11 melee damage with a fast attack rate...