Uncommon/Rare Weapon (Flail), Requires Attunement

Strikes of Rot. Once per turn, before you make a melee attack with this weapon, you can choose to take a -5 penalty to the attack roll. If the attack hits, the target must make a DC 12 Constitution saving throw, taking 1d4 poison plus 1d4 necrotic damage and gaining a -1 penalty on their attack rolls and saving throws made until the end of their next turn on a failure, or only half the damage and no penalty on a success.
Creatures immune to Diseases automatically succeed on the saving throw. 

Noxious Fumes (Recharge 5-6). You can use your bonus action to make all creatures within 5 feet of you roll a DC 12 Constitution saving throw, taking 1d4 poison and 1d4 necrotic damage on a failure. 

• Whenever you roll a 20 on your attack roll with this weapon, you immediately recharge this feature. 

Secret (For GMs only)

Pathogen Mutation. If a creature succeeeds against a saving throw imposed by this weapon by 12 or more, the diseases inside the weapon begin to evolve.
After this happens for the third time with a single wielder, the weapon becomes more powerful while that wielder is attuned to it:

• You gain a +1 bonus to attack and damage rolls made with this magical weapon
• Attacks made with this weapon deal additional 1d6 necrotic damage 
• The DC of this item’s features is increased to 15
• Once per long rest, you can cast the Contagion spell (DC 15) using this item, but it ends after a single successful saving throw by the target, and lasts for the maximum of 1 minute.