This has to do with Scythe of the Wretched, an artifact equipment that gives the equipped creature +2/+2 and says that whenever a creature that is dealt damage by the creature equipped with this card is put into a graveyard, you can return it to the battlefield under your control
The design team (Aaron Forsythe, Richard Garfield, Mike Elliott, and Tyler Bielman, led by myself) approached Dimir as the "library guild," much as we approached the Golgari as the "graveyard guild." As I explained above, this comes about because blue and black have more interaction with the library than any other two-color combination
Every card with the ability felt kind of unreal and powerful, even terrible 3/3s for six
And sorry Ulamog, the Ceaseless Hunger: Youre only a 10/10, and you must be this big to enter the list