Hand of Fate 2

In the new endless mode the game mode itself is broken down into smaller chunks called Adventures. The early Adventures are two to three "Boards" of cards and are fairly quick to complete. The Adventure is counted as complete when you kill the boss monster for that Adventure, all the trials resolve themselves and you return to the City for your next adventure.

This achievement requires that you complete 50 total adventures over the course of all your Endless Mode runs. It should just be a matter of time, good luck out there!