Unfortunately, Magic: The Gathering did rely on a number of limited releases, making some cards extremely difficult to get
Delirium If there are four or more card types among cards in your graveyard, instead search your library for any card, put it into your hand, then shuffle
The really useful ones are the mass sacrifice sources
Generally you want to make sure you have a two or three-color identity for a theme like this, and remember that the majority of your good enablers will be in Grixis colors: Brallin, Skyshark Rider plus Shabraz, the Skyshark Raffine, Scheming Seer Chainer, Nightmare Adept Rielle, the Everwise Soul of Windgrace Amber Gristle OMaul plus Agent of the Shadow Thieves (or Criminal Past) Slogurk, the Overslime Despite their contrarian approach to the Commander metagame, discard decks are capable of some truly disgusting lines and offer a huge range for individual expression
Having all of these different options on one permanent is simply too good to pass up and should definitely see more play than just 11% of Isshin decks
Before I get into my usual rundown of the Cube by color, lets talk about the significance of the Cubes color restriction