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For some reason I always forget this Mon exists and it irks me. Heck, two of my favourite Trainers in fan games (one is a massive spoiler, the other one would get me roasted to high hell) own one and I can never recall that fact without looking it up. It's a shame, because it seems like a great Mon.
According to bulbapedia: "Dhelmise prefers large prey, such as Wailmer and Wailord. The heavy anchor can be swung with enough force to knock out its prey in a single blow. The victim is then enveloped in seaweed, which can be stretched hundreds of yards, and its life-force drained."