Warhammer Age of Sigmar: Storm Ground

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


Once unlocked, Edit/Equip your Knight of Shrouds unit and go to a Skills slot. Scroll until you find "Summon Winged Nightmare" and equip this to the Knight of Shrouds.

Now start another automatch game and keep ending turns until you accumulate 7 of the blue resource (Aethyr?). You'll then be able to use the 'Summon Winged Nightmare' 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.