The Dead Warrens

Locating the wheelbarrow trail is a DC 15 Perception check, as is finding the secret trap door within the mausoleum. Identifying the derro footprints is a DC 25 Nature check.

Derro do not exist in 4th Edition at present. This isn't a big deal at all, since they can be easily and effectively represented by reskinning kobold stat blocks. If you'd really like to customize them to contain more derro flavor, consider adding some powers from the Those Who Hear monster theme from the Dungeon Master's Guide 2 (page 120).

Be sure to increase the physical size of a number of the rooms in the Warrens. A few of the areas (especially D4, D6 and D8) are far too cramped to allow for the sort of dynamic combat the game is suited for. Discovering any of the crawlspace secret doors in the Warrens requires a DC 18 Perception check.