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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It would be especially cool if it's exotic perk was a special melee attack that does more damage, like te C-90 Bo Rifle in Star Wars Battlefront. This would be good in Destiny 2 because it now takes 3 melee hits to kill someone in PvP in D2.