Blue will bring the ability to draw enough cards to keep going and return annoying permanents to your opponents hands, black brings with it graveyard tutors and the ability to return permanents from your graveyard to the battlefield and green gives us removal pieces to deal with some common graveyard hate effects, plus some of the heaviest hitters in the format that are worth reanimating
If you put +1/+1 counters on five Dragons this way, you win the game
Turn 1: Noble Hierarch
Not only that, but for four mana, two of which is green or blue, you can put the top two cards of your library into your graveyard, then an opponent gets to choose a nonland card to take from your graveyard and put into your hand
Deaths Shadow first started seeing play a few years ago in a very different type of deck in Modern, called Suicide Zoo
Prosper, Tome-BoundProsper, Tome-Bound is obviously the king of Rakdos Treasure builds, and I think Kellogg will end up in the 99 of those more often than leading his own deck