Henadic Theologian
Legend
It's two keywords: "Dungeon" for the new card type, and "Venture" for how we interact with them (which remains a mystery). Very old school D&D sounding, intrigued to find out about actual mechanics. We now know Dungeon cards are something that can be completed, after a venture (worth noting that adventure, key yo the name of the Set, is literally "venture into").
I think Charlaquin is right, it's actually a Keyword action like Amass/Scry/Surveil, not a Keyword ability like say Haste/Double Strike/Life Link/Death Touch. The first describes a particular action the card takes, and the other describes an always on ability.
So Venture Into the Dungeon and Complete a Dungeon are Keyword Actions that occur or have occurred.
Dungeon is likely just a new Card type or super type or subtype.