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