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Well... Chainsaw is considered an exotic, and Longius is a special dungeon reward. There are lots more uniques of other types which drop more. I've seen the Cleaver once ever, on a Shotgunnery run, straight after beating AoB. 

[Necronomicon]

Necronomicon
One-use item, probably either
-drains the life from all nearby monsters and heals you up to 300%, which will decay as normal, down to 200% for Badass.
-draws power from all corpses on the level, and heals you the same way, or possibly makes you invulnerable (mortuary anyone?)

Since the Butcher's Cleaver is in there...
Hellforge Hammer
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Damage - 15d4
Inventory description - The hammer of Hephaestus, said to shatter the very soul of evil.
Pickup - The power of hell flows into your hands.
Acc - + 6
Firespeed 1.8s
Normal fire - smash a foe with the hammer
Alt-Fire - Shatter the very earth! Aim it like a normal weapon, causes damage (less than attack) in a line, and turns the tiles struck into lava, including walls, tires you out.

Found in a unique level perhaps, the Hellforge - You must defeat Hephastus (a powerful melee-baron), and can choose to either shatter the hellstone *, and gain some random loot, mostly upgrade kits (for non-melee chars) or keep the hammer.