Tiamat's Dimension is an interdimensional realm occupied by the god Tiamat.
In the springtime, Dana Barrett was on the phone with her mother when she was whisked from her kitchen to Tiamat's Dimension and interrogated by Tiamat about the nature of her time as host of Zuul. Once Tiamat was done toying with Dana, she was back in her kitchen. After being lured to Dana's apartment, the Ghostbusters took the stairs to the roof and instead found themselves in her dimension. After they Crossed the Streams on Tiamat, the Ghostbusters found themselves back on the apartment roof. After Tiamat sensed Gozer's presence in Ray's mind, she crossed over from her dimension.
From her dimension, Tiamat used a well and observed the prime dimension Ghostbusters' meeting about their missing ghosts with teams from various parallel dimensions. Peter signaled the end of the meeting and the start of the retrieval missions with a warning to be careful since they don't know what they're walking into. Tiamat laughed and remarked truer words were never spoken. Tiamat observed Central Park of 00-D and was amused that the Headless Horseman chose Peter Venkman of Dimension 68-R, of all people, to possess instead of one of the local inhabitants. Then she saw Abby Yates of 80-C being chased around by the Moose Ghost in the Helsingor Brewery of 75-B. Tiamat was delighted the others in the multiverse would hear of the Ghostbusters' travels and even The Predators would panic, resulting in chaos. She admitted she was glad she let the Ghostbusters live a little while longer. Koza'Rai, Tiamat's father, made contact. Koza'Rai warned her she would regret it and reminded her how the Ghostbusters dispersed Gozer's essence, bound him to an inescapable Nether realm, and trapped The Collectors. He found them far more resourceful than she gave them credit for. Tiamat quipped she gave them all the credit. Koza'Rai scolded her for her tone. Tiamat admitted it was the capture of The Collectors that caught her attention in the first place as well as their ability to travel the multiverse would serve her goals. She continued and listed off how their impulsiveness, destructive tendencies, and the supreme ease of manipulating them combined with the smallest of encouragement would spread chaos faster than any servants she ever had in her employ. Koza'Rai countered they weren't her servants and would be her undoing. Tiamat dismissed him and declared she knew what she was doing. She turned to watching the Circus Dimension next.
Aside from a throne, the dimension is a void.