There should be some exotic gun with a blade on it like read death but when you melee at critical health it does a lot of extra critical damage.
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The Monte Carlo has a perk just like your describing. Teaming it up with a Pair of Exotic Warlock Gloves that tunes your punch even more.. I'm giving out my referral link for people to join me in the Keystone closed Alpha. Its the new game by the makers of Warframe. If anybodys intrested. Please feel free.. use my referral link to join up for this weekends Alpha trial. https://www.playkeystone.com?referrerId=5924a93e4cde6fd91b4c3102