The Walking Dead set of cards was originally released as a part of a timed-exclusive Secret Lair, and remained the only way to get them until Wilds Of Eldraine almost three years later, where they were reprinted with in-universe Magic characters in Magic settings
Regardless, the more interesting part of Mondrak is his role as a sacrifice outlet
I know power creep is a problem in Magic, but you dont expect a legendary mythic to be superseded by a nonlegendary rare just a few months after release
If an effect allows you to search your library for a card with mana value 4 or less, you can't find Bottomless Pool // Locker Room
That creature is now your Ring-bearer. They can only be blocked by creatures with a lower power
The end result is a deck that is low to the ground and casts lots of spells per turn, swinging in each turn to chip down life totals and then closing out the game with a combination of burn and creature combat