El Devorador is the monster at the centre of The Vanishing, the free Stranger Things crossover mission. It is the Demogorgon from the show, renamed by the Soviet Lock Project that studied it.
You only face El Devorador in The Vanishing. Start that mission to see it; it never appears in the open world.
Come prepared for a short, scripted fight rather than a roaming hunt. Bring fire for the Kukly that surround the mission, since they are weak to it.
El Devorador does not survive The Vanishing.