Me being Hungarian, was expecting somewhat bigger challenge but actually it was very easy.
Obviously, start as Álmos in 867 The Great Adventurers.
Immediately go to the Decision tab and choose "Prepare to cross the Carpathians". This would give you a Casus Belli against any ruler having land in the area of Pannonia. Declare war using the Casus Belli "Migrate to Pannonia" that would give you 5000 extra troops.
Raise all your regular armies and keep them near your capital for protection, while use the 5000 to win the war against your chosen enemy. I choose Bulgaria and had to win 3-4 battles and capturing 6-7 counties, but can imagine it would work with any of the smaller duchies within Pannonia that can be broken with one siege.
Once the war has been won, Pannonia will be formed with some further consequences that you shouldnt worry now. Go to the Faith tab, choose Catholics and convert. Achievement pops.