He's a cheap threat that can grow throughout the game, draws you a card and can't be blocked whenever you play Mishra's Bauble, The One Ring, or any legendary spell, and doesn't die to Orcish Bowmasters
Really, resolving an Underworld Breach allows you to win any number of ways
Sword of the Animist Sword of the Animist is one of the best ramping equipment cards in Magic because you only have to declare an attack with the equipped creature to search for a land rather than having to land combat damage
Lost Caverns unabashedly pulls from several Latin American cultures across different periods of time
Spore Frog + Kami of False Hope The strength of Spore Frog and Kami of False Hope lies in how easy they are to recur
Nethroi, Apex of Death Nethroi, Apex of Death has a pretty good ability to reanimate creatures, and it can be relatively cheap depending on the mutate creatures you put in the 99