It started from developer. To get it you should play in co-op with infected players.
Complete the Unbalanced Build challenge
Kill the Spider boss without taking any damage
Die in the Viscera location
Kill 100 enemies with buzz saws
Kill two enemies simultaneously with a buzz saw
Kill 100 enemies with a Double Buzz Saw
Complete the "On the Razor's Edge" challenge
Explore all rooms in 3 consecutive locations
Kill the Worm boss in the Secret Lab
Kill the boss in the Organism location
Kill the Wasp boss
Kill 10 enemies with a Wasp
Kill the Dog boss
Resurrect an ally on your team 3 times
Resurrect 3 different allies in the same room
Complete the Furnace without falling below 50% health
Pick up 3 mutations in the Secret Lab
Complete the Viscera location
Complete the "Labyrinth" challenge
Kill 10 enemies with a flying barrel
Unlock all doors in all locations
Upgrade the store 1 time
Upgrade the store 10 times
Kill the Tenheads boss in the Furnace
Take an elevator down to the Organism location with a team of 3 friends
Unlock a new mutation in the Viscera location
Go down to the Catacombs
Kill the Brain boss
Kill the Worm boss while the Rage skill is active
Complete the "Survival" challenge
Find 2 stun guns in a single run
Kill 4 enemies in one room without hitting them directly
Kill all enemies in one location without hitting them directly
Resurrect with the help of an Embryo 5 times
Unlock a new mutation in the Factory
Unlock a new mutation in the Secret Lab
Unlock a new mutation in the Furnace
Unlock a new mutation in the Freezer
Die alongside one or more players
Kill 3 enemies with one hit
Complete a successful run with four players in co-op
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