Unlimited Bat Zombies 1.2! (Legacy)

Get a Desecrated Tomb into play with a Gravecrawler and Phyrexian Altar on the battlefield as well. Sacrifice Gravecrawler for Phyrexian Altar's mana ability and use that mana to cast Gravecrawler from the graveyard. As Gravecrawler leaves the graveyard, Desecrated Tomb will trigger, producing a 1/1/ flying bat token. Make sure you have another Zombie on the battlefield so that Gravecrawler is capable of casting from the graveyard. Carrion Feeder's sacrifice ability will work as a nice proxy for Phyrexian Altar until you can get Phyrexian Altar out, but it won't provide and infinity loop. If you get Bridge From Below into the graveyard with either Entomb or Cryptbreaker's discard ability, Gravecrawler will create a 2/2 Zombie token as it enters the graveyard.

Once you get Gravecrawler back onto the battlefield, you can perform the Phyrexian Altar sacrifice/mana/recast combo infinite times because Phyrexian Altar doesn't tap. Every time Gravecrawler enters the graveyard, it creates a 2/2 Zombie with Bridge from Below. Every time it leaves, it creates a 1/1 flying bat with Desecrated Tomb.

Lockdown the opponent's mana with Contamination and turn your bats into Zombies if you happen to get a Conspiracy out. Endless Ranks of the Dead will provide a healthy supply of Zombie tokens each upkeep if you don't manage to set up the infinite Phyrexian Altar in the early game.