You get... DEVOUR BODY

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You get this weapon from defeating Devour Man ([link]). Megaman gains a bit of weight, but damage done to him is reduced by half, though his speed is reduced by half as well, and he can't slide. Each hit also heals him, depending on the amount of damage he took. (In-game, for example, if Megaman is hit and 4 bars of health is dealt (initial is 8 bars, but gets halved, so 4 bars), he heals back 2 bars.)
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Sprites used from Sprites' Inc
Concept from my evil mind...