"A regression to ancient forms? A glimpse of future evolutions? Or a fleeting alteration of the present?"
—Vorel of the Hull Clade

Biomass Mutation

Creatures you control have base power and toughness X/X until end of turn.

Duskmourn: House of Horror Commander

Vault 87's Evolutionary Experimentation Program exposed subjects to an airborne version of FEV. Rapid physical enhancements accompanied sharp mental regression, and the oldest mutants would become almost feral.

Fallout

Biomass Mutation overwrites any effects that set the power and/or toughness of a creature you control to a specific value. Effects that modify power and/or toughness but don't set them to a specific value (like the one created by Giant Growth), power/toughness changes from counters, and effects that switch a creature's power and toughness will continue to apply. For example, say you control a 1/1 creature that's getting +4/+2 from an Aura, has a +1/+1 counter on it, and is affected by an effect that switches its power and toughness. This creature is 4/6. If you cast Biomass Mutation with X equal to 4, the creature would become 7/9 (4/4, +4/+2 from the Aura, +1/+1 from the counter, then switch power and toughness).

Choosing 0 as the value for X will likely cause creatures you control to become 0/0 and be put into the graveyard.