Gunpowder

Gunpowder is a unique type of flammable creep hazard added in Fiend Folio that is primarily created by various enemies and bosses found within the Burning Basement, or by Isaac with the Modern Ouroboros.

Gunpowder acts similarly to black creep, slightly slowing down the movement speed of Isaac or affected enemies. However, when exposed to sources of fire or explosions, it will ignite, creating flames that will either hurt Isaac or hurt other enemies that can be affected by it. This will spread to all other piles of gunpowder nearby, and the flames will linger for several seconds, which can happen even after the gunpowder itself disappears after enough time passes. Having the ability to fly or the Callus trinket will negate the movement speed penalty for standing on gunpowder, but it will not protect Isaac against the flames created by ignited gunpowder.