The lowest level of the city, the Catacombs were located in a tunnel network under the granite foundation of the city. Unlike the rest of Djerad Thymar, the tunnels weren't created by the dragonborn. The tunnel network had existed in the region before the Spellplague, as part of a ruined Untheric mausoleum. The djerad fell above the ruins in wake of the Spellplague, and the dragonborn remodeled the tunnels to suit their needs when they constructed Djerad Thymar.
The most transited sections of the tunnel network had various chambers and rooms that served as a storage area for military equipment and siege supplies, or to fulfill many other mundane purposes as well. The dragonborn also used a section of the network to serve as the graveyard of the city. The graveyard was composed by a series of burial chambers, each clan having their own private sections of the network for their own dead. Those areas were filled with sepulchers for mummified dragonborn who died on Toril, and murals known as the "Roll of the Lost", with names in draconic of dragonborn ancestors who died in Abeir or weren't transported to Faerûn during the Spellplague. The lowest section of the tunnel had a chamber that served as the tomb of a bearded warrior of Toril but claimed as clan-kind of all dragonborn, whose tomb had existed in the original Untheric ruins who stood in the area before the construction of Djerad Thymar.
Not all the tunnels under Djerad Thymar were used for those functions, however. In the desolate sections of the tunnel network, the poor, as well as outcasts and criminals, had their homes and lairs. Those sections of the network were ill-reputed due to rumors of illegal activity or undead haunting them, and weren't patrolled by thymari authorities. 
Like the Market Floor and the City Bastion, the Catacombs's walls were embedded with magical glowing balls that provided sources of illumination. However, the abandoned sections of the Catacombs were left in disrepair and most of the magical lights had faded out, either because of time and neglect or because someone had extinguish them.
- Gem Assayers Union headquarters
- The little known guild had its main headquarters in the Catacombs of Djerad Thymar.
- Platinum Cadre headquarters
- The Platinum Cadre had their headquarters in one of the abandoned sections of the Catacombs.
- Bruce R. Cordell, Ed Greenwood, Chris Sims (August 2008). Forgotten Realms Campaign Guide. Edited by Jennifer Clarke Wilkes, et al. (Wizards of the Coast), pp. 186–187. ISBN 978-0-7869-4924-3.
- Keith Symcox (2009-12-23). Death before Dishonor (TYMA1-04) (ZIP/PDF). Living Forgotten Realms. Wizards of the Coast. p. 6. Retrieved on 2017-09-08.
- Erin M. Evans (December 2015). Ashes of the Tyrant. (Wizards of the Coast), p. 291. ISBN 978-0786965731.
- Erin M. Evans (December 2015). Ashes of the Tyrant. (Wizards of the Coast), pp. 291–294. ISBN 978-0786965731.
- Richard Lee Byers (2010). Whisper of Venom. (Wizards of the Coast), p. 33. ISBN 0786955619.
- Erin M. Evans (December 2015). Ashes of the Tyrant. (Wizards of the Coast), p. 57. ISBN 978-0786965731.
- Erin M. Evans (December 2015). Ashes of the Tyrant. (Wizards of the Coast), p. 359. ISBN 978-0786965731.
- Erin M. Evans (October 4th, 2016). The Devil You Know. (Wizards of the Coast), p. 270. ISBN 978-0786965946.
- Richard Lee Byers (2010). Whisper of Venom. (Wizards of the Coast), p. 108. ISBN 0786955619.
- Living Forgotten Realms, TYMA1-05 From One Dwarf to Another, Appendix 3.