Tombs of the Fallen is free content introduced in update 1.4.0.

Adventure calls and those curious enough to answer will find the all-new Tombs of the Fallen. In these puzzle-focused tombs, you'll delve into vast, hidden subterranean caverns to be rewarded with tales of mystery, history, and irresistible loot. Use your parkour abilities to make your way through these tombs filled with danger -- watch out for traps and poison! Observe and explore your surroundings, and solve environmental puzzles to acquire time-forgotten treasures.

Additional Tombs of the Fallen were introduced with update 1.6.1.