Warhammer Age of Sigmar: Storm Ground

To unlock the Great Unclean One in multiplayer (the automatch mode), you first need to reach faction level 30 for the Maggotkin. In my experience, this takes roughly 60 wins if you are using the method outlined in the guide here:


Once unlocked, Edit/Equip your Lord of Plagues unit and go to a Skills slot. Scroll until you find "Great Unclean One Summon" and equip this to the Lord of Plagues.

Now start another automatch game and keep ending turns until you accumulate 10 of the blue resource (Aethyr?). You'll then be able to use the 'Summon Great Unclean One' ability. It takes a few turns to pop out though, so end your turn another 2-3 times and it should summon, and your achievement will then unlock.