I Am Plague is a dark fantasy incremental roguelite in which players build a rat horde to climb an underground fortress. Combat is automatic, with outcomes determined by mutations, abilities, and their synergies. Players select mutations, visit shops, and choose branching routes through dungeon floors and text-based events. Flesh and skulls gathered during runs fund permanent upgrades and unlock new mutations at the Tree of Flesh and Tree of Bones. The full game is planned to include over 200 floors, more than 50 mutations across several rarity tiers, and increasingly dangerous enemies and bosses.