This card costs four mana, is instant and has bargain, which means you can sacrifice an artifact, enchantment or token as you cast it for a better effect
Allowing these Tyranids to replace themselves if you spend enough mana means that you can keep your hand full as you enter the later game and continue to bring new threats down to attack your opponents
9 Unnatural Growth For a mere five mana, Unnatural Growth is a green enchantment that simply reads, At the beginning of each combat, double the power and toughness of each creature you control until end of turn. Were not really sure what else to say here, except that you should probably make sure you can give your obnoxiously large creatures trample
3 Traditional foil uncommons
An activated mana ability is one that adds mana to a player's mana pool as it resolves, doesn't have a target, and isn't a loyalty ability