DANGER! Dark Rainbow!

The main focus is using Magical Merchant to mill as many cards from your deck as possible then summoning Rainbow Dark Dragon and banishing as many remaining DARK monsters from your graveyard to increase his attack to a ridiculous extent.
To make the most out of Magical Merchant's ability, Danger! Response Team is returned to the bottom of the deck by discarding a Danger! monster before flipping Magical Merchant face-up.

Other cards like the Danger! and Shaddoll monsters help cycle through the deck and get monsters out for Extra Deck card resources. Destrudo specifically is used to both Synchro summon Black Rose Dragon and as a way to get that one more monster you need on your field for a Link summon. Dark Grepher and Armageddon Knight are used to help trigger graveyard effects of other monsters. Broww is used solely as draw power off Danger! effects and Marron is used to ensure you don't deck out.

There's a OTK combo which mainly relies around emptying your deck into the graveyard with Magical Merchant and then summoning Rainbow Dark Dragon and attacking for lethal on the same turn. To support this combo there's a couple of contingency plans for when things aren't optimal:

• If Rainbow Dark Dragon is in the GY: Wee Witch's Apprentice can be used to recover Rainbow Dark Dragon from the GY.


• If you Need Opponent's Cards Gone: Black Rose Dragon is used to wipe the field to get rid of any annoying cards.

In the "worst" case scenario (Not really the worst case...) where Rainbow Dark Dragon is in the graveyard and your opponent controls cards that'll stop your attack from going through, you can use both Wee Witch's Apprentice and Black Rose Dragon to return a copy of Rainbow Dark Dragon to the hand and wipe your opponents field before summoning the Rainbow Dark Dragon and banishing all your remaining monsters in your GY to gain lethal attack. (This relies on there being enough cards in your GY though, so keep that in mind).

 

I'm more than appreciate feedback and suggestions, I just thought making a deck around Rainbow Dark Dragon would be a fun thing to try :)