
What Necrochasm could have been (Destiny)
Cursed. Grows stronger by feasting off fallen enemies. Why couldn't Bungie have rolled that into the exotic? Imagine the main perk:
You cannot un-equip Necrochasm while in a darkness zone or while alive in crucible. Health regeneration from any source is disabled. You are healed 5% of damage dealt, up to 200% health. Weapon damage scales with your health.
So, while at 200% health, the gun does 200% damage. BUT, while at 30% health, the gun does 30% damage. This creates a positive feedback loop. The stronger you are, the more damage you do and the more you can heal. The weaker you are, the less damage you do, so it's even harder to heal.
You can only change out the gun while dead in crucible or while outside a darkness zone, to keep you from just changing guns to heal.
It'd be a high risk high reward gun, especially in crucible. If you health goes too low, you'd be dead in the water since your damage would be pitiful. But stay strong, and you'll have a beast. Secondary, Heavy, and supers would still work, but you can only heal with damage dealt using Necrochasm.
Something like that.

Meh
You deal less damage as you are getting shot? I don't think that would work very well at all. At the very least it would be time-consuming to balance.

What Nechrochasm also could have been
Imagine this: the gun shoots vodka infused gummy bears.
They don't do much damage individually, but have a stacking damage over time effect so they still kill as quick.
And, instead of killing, they just make the guy pass out and pee himself. BUT the pee is toxic, so it kills all the other nearby dudes.
Perks: Golden Corral gummy bears. The last half of the magazine shoots gummy bears that have been sitting outside at an all you can eat buffet next to the ice cream for weeks and are hard as a rock. They do more damage, but don't deal DOT.
When you reload, it makes kitten meowing noises.

What Nechrochasm also could have been
This could actually be a thing in Borderlands.

We'll call it: the Nyancatchasm.
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Meh
You deal less damage as you are getting shot? I don't think that would work very well at all. At the very least it would be time-consuming to balance.
I'd be interested to try one out who's damage was the inverse of my health. The question is, how do you balance a gun like that and at what point do the two lines intersect? 100% health? 50% health?

What Necrochasm could have been
TBH, I think the concept sounds fun. Use of cover and surprise attacks would be crucial.
I'm sure the exact numbers would need tweaking for balance, but I don't really see a problem with the basic function.

It's actually a 9th degree polynomial function. Here you go.

It's actually a 9th degree polynomial function. Here you go.
Batman hanging upside down? ;)

What Necrochasm could have been
Meh. The extremes are far too extreme. After a few hits the gun is suddenly doing +33% damage... that's a Weapons of Light level increase just for landing a few shots! The interplay between doing damage and receiving damage might actually be pretty interesting to ponder but all someone really needs to do is get in a little damage and they are suddenly god-like. Remember, Pulse Rifles moved from unused to potent with a mere +9.7% increase in damage... and you're setting the top end boost at +100%?! While the player is twice as tough?! It would take a rocket or a super to kill you... if the enemy could get close enough, which would be one heck of a big if.
A maximum of +10% boost to damage and health might start to approach reasonable levels. +100% is just crazy talk and would be game breaking.

TF2 did this...
There's a pair of boots the soldier wears that allow your rocket launcher to deal more damage the closer you are to being dead. It had it's day, some good side effects like rocket jumping and then dealing extra damage from unexpected positions. But IIRC it was balanced enough that it wasn't any more or less powerful overall than anything else.
You know, like Gjallarhorn. ;)

Woops, nevermind, that already exists.
Leave it to f%&$ing Garry's Mod.
So this, but with alcohol soaked gummy bears, Funkmon?

Dead on.
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What Necrochasm could have been
A maximum of +10% boost to damage and health might start to approach reasonable levels. +100% is just crazy talk and would be game breaking.
In PvP, you take a lot of damage very quickly. It would not take much to bring you back to normal. Plus, you are forgetting about the opposite end: where you take so much damage that you now have a pea shooter.
It would take time to build to 200%. 5% of 14 is .7. That's how much health you'd gain on the first shot. You are looking at putting 50 something headshots just to get to 200% in the first place. Can you do that without taking damage? If you can, then you definitely deserve the boost, and the opponents deserve what they get for letting you get there.
It balances out. Remember, Necrochasm at 100% damage sucks ass.

Thrall explosions trigger off any hit- now fixed
Increase rate of fire, larger mag, and done. Let'er rip boys!

I'm guessing it should be called...
...the Necrogasm?
Sorry, everyone. Truly.
YEEEEAAAAAAAAH!
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Batman surrendering!
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I've always called it that.

I've always called it that.
Since I just joined this forum, I couldn't possibly have seen that.

Hey...you're trying to trick, me aren't you?
Since I just joined this forum, I couldn't possibly have seen that.
I don't really have one that fits perfectly. Damn.

Hey...you're trying to trick, me aren't you?
Since I just joined this forum, I couldn't possibly have seen that.
I don't really have one that fits perfectly. Damn.
Hah, still made me laugh aloud!

What Necrochasm could have been
Just read the grimoire card.
The result was a weapon that would feed on its owner’s aggression—reaching further when angry eyes drew focus, its hunger rising as it tore through bone and flesh.
Replace rangefinder with this perk:
This weapon grows more accurate and does more damage the longer it is fired (up to ~200%)
If that doesn't reward mindless aggression, I don't know what does.