I've had an interesting idea on creating a boss for my Players to fight for a One-Shot. My idea is that this boss is the master of making creatures decay rapidly until they turn into dust and bone. Another thing the boss can do is collect souls when it kills and use it to power himself up.
I've attempted to look at some creatures from the Monster Manuel but turned up with noting. I want this boss to be balanced (obviously), so I wanted to turn to this Discussion before going Homebrew. Does anyone have any ideas? Is there a creature that can do something like this already that I can use?

The first two things I thought of are the spells wither and bloom, and Abi-dalzim’s horrid wilting. 
I bet there’s other things out there, hopefully others will chime in. 
Also, while it’s generally not good to use PCs as bad guys, you could take a look at spore druids, they might give you some ideas. Or if you can find some old dark Sun stuff, check out defiliers.

Not that I'm Aware of but go for it dude! If you want to make it scary try adding a story to him! Make the PC's see flashes of him here and their. Give them enough details that they know some stuff about the creature but not a lot. Make his entrance scary!

I'd say homebrew away:) I usually like to get a basic concept that I know works and is balanced and then "flavor to taste 'Lichen Lich' from Candlekeep's 'Xanthoria' would be a great starting point. 
Otherwise since I'm kind of getting druidic vibes I'd also check out Pointy Hat's Druid Lich Guide

The closest thing I can think of is how a mummy's touch curses a creature and causes their skin to dry out and spread the longer they're cursed.

I'll poach these links form another thread I commented on:

Here a nice little article about Zuggtmoy: https://chriswymanwriting.medium.com/roleplaying-zuggtmoy-751b68657a07

You could also look into Moander; former God of Rot, Decay and Corruption. Both Moander and Zuggtmoy share the same sort of world view and domains of powers. Link to Moanders Wiki here: https://realmofadventure.fandom.com/wiki/Moander.

Both these two could work as a patron if you go down the Cleric, Druid or maybe Paladin or Divine Soul Sorcerer style boss.

Alternatively, there is always the trusty old Hag Coven, their champion is going here and there, corrupting people and snagging their souls which the coven intend to use to evolve their Fey forms into Fiends and transform into Night Hags. You could then have various trinkets and baubles which house the souls of those the champion corrupted and these can be used to mimic the effects of certain spells. This could give you plenty of leeway as a DM as to what abilities the Boss has. Give him a weapon which has a effect similar to that of the Soul Cage spell so he can automatically trap the souls of people he kills and store them in one of his trinkets.

Blight and Chill Touch might be useful spells to look at for that decaying style ability.

Thank you for all the tips. I have an idea on how I can go about making him.
If anyone else can come up with more ideas. I'm all ears.
Thank you

I have a few ideas 

• If you wanted to extend it to items as well as creatures there is the rust effect of rust monster and corrosive form of the black pudding  you could easily make an organic version that applies to materials like wood and leather  and maybe deal damage if they strike with natural weapons/unarmed.  Keep in mind players don't like items being damaged so I'd telegraph it and probably make it something they have to learn to overcome as opposed to something they have to deal with in the final battle. 
• If you want a creature that radiates corruption and unlife then there are the mechanics of the sibriex with it's contamination aura and warp creature. Of course instead of mutating and converting to a demon, you would have them turn to dust on death.
• There's also the channel negative energy of the mummy lord that prevents healing. You can also put the preventing healing on it's attacks or an aoe as well. 
• antilife shell prevents living things from entering  which can be described as a repelling on life 
• negative energy flood and finger of death could be adapted to  create an ability that rots a person down to a skeleton under their command.
• A cloud of death is also pretty scary, adapting the mechanics  of cloudkill to be say a necrotic cloud that prevents healing, applies the exhaustion effect of warp creature or decays organic material could be really terrifying to a party.  instead of an aoe you could also make such a death cloud a movement ability like beledros witherbloom 
• The vecna the archlich stat block also has some neat tricks like multiple reactions, decaying rebuke 

The best ideas come from the best designers.  Check out Flee Mortals by MCDM, in particular the stuff on solo monsters https://www.youtube.com/watch?v=W0npb3FSWw0

Disintegrate too