The exotic trait should be that you can overcharge the heavy for more damage & an increased explosion radius, but consume more ammo the longer you charge,
I know it could be a problem, but I would want this to deal an insane amount of damage, like nuke gm champions & deal raid level dps.
If a catalyst was allowed, I would say shattering blade could fit the idea.
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What does shattering blade do? An exotic heavy sword sounds right on time. And it wouldn't be the first sword to nuke champions. Can still remember the first time I saw lament and I thought the people using it were cheating.