MonsterEnvy
Legend
I read Madness at Gardmore and Ruin is a sequel.
It assumes that the entire adventure happened, with the Adventurers collecting all the cards and defeating the big monsters like the Beholder and Oni. But when they got to Mekkaleth the Young Red Dragon the final boss in the catacombs, they were defeated and driven away with the Dragon claiming their cards and gaining a complete Deck of Many for his hoard. Mekkaleth then outgrew the Catacombs as he became an adult, so he moved to the ruins above with his hoard including the deck.