Tiamat is a primal goddess who banished Gozer from Earth in ancient times.
Ghostbusters: The Video GameEdit
In 4th millennium BC, the Cult of Gozer, or "Gozerians", consisted of a large Sumerian sub-culture and was engaged in a long, protracted war with the followers of Tiamat. Eventually, Gozer and its followers were defeated and Tiamat banished Gozer from this world.  
There are several conflicting legends in the mythology surrounding Tiamat. In some, she is a deity. In others, she is considered the embodiment of chaos. This version was killed by Marduk and he used her corpse to build the world. One aspect the legends remained consistent, Tiamat was the sister and rival of Gozer. Tiamat is more powerful than Gozer.  She is not bound by rules and as the goddess of chaos, the only constant has been her feud with Gozer.  Even the followers of Tiamat and Gozer had been at odds.  It is believed she or her followers were responsible for banishing Gozer from this dimensional plane. 
Tiamat took an interest in Earth after the Ghostbusters defeated her brother and had the power to suddenly rip holes in the fabric of the universe when they escaped from the Collectors' Limbo. She decided it was somewhere she would go to to play. She was overcome with curiosity and wanted to toy with them.   She detected Eduardo Rivera in her realm and tasked him with delivering the message of her impending arrival. Tiamat blasted Eduardo and he soon woke up back in Ray's Occult Books. Eduardo asked Ray Stantz and Kylie Griffin if they knew what a Tiamat was.
Tiamat was present at the Field Museum of Natural History when the Chicago Ghostbusters examined the World of Gozer exhibit on Ilyssa Selwyn's behalf. She used her powers to activate a Gozerian artifact and influence the creation of the Two-Headed Man-Eaters of Tsavo. Tiamat left "Beware" in Gozerian on the artifact as a warning. After the Chicago Ghostbusters destroyed the artifact, they left the exhibit with Selwyn unaware Tiamat was observing them from the second floor.
A couple weeks later,  in the springtime, Tiamat wanted to start her "game" by having everything her brother Gozer had, including proxies. She manipulated Louis Tully's dreams and instilled an overwhelming desire to return to New York City. She then summoned Dana Barrett to her realm. Dana tried to run but was captured by one of Tiamat's Terror Birds. Tiamat then asked Dana if her being the host of Zuul was fate or a coincidence. Dana believed it was never supposed to happen all over again. Tiamat then retorted how she knew that and returned Dana back to her kitchen. Tiamat's presence on the physical plane began to induce strange phenomenon such as Blood Rain and gravitational anomalies. 
Tiamat had Dana restrained and her female Terror Bird take possession. Tiamat then took on the form of a blonde woman and approached Louis at a bar. She soon became annoyed with Louis' "rigid mind" and ripped her face off, revealing her true form. Louis passed out. The bartender wanted nothing to do with the encounter. Tiamat grabbed Louis and took him back to Dana's apartment where he was possessed by the other Terror Bird. Once the possessed Dana lured Peter, and by extension the Ghostbusters to Dana's apartment, Tiamat had her servants declare she was ready to see them. Reenacting the Ghostbusters' first encounter with Gozer, Tiamat forced them to take the stairs. At some point, it appeared Tiamat's presence caused a simple minor chronal anomaly. Future versions of the original four Ghostbusters appeared before the team and presented cryptic advice on dealing with Tiamat - that while she was more powerful than Gozer, she had a different agenda and that could be used to their advantage. In addition, they reiterated her addiction to chaos and hinted at what one needs for chaos. Winston then learned from his future self he would be the one to figure it out when the time was right. After making it to the roof, walking up her temple, and encountering the Terror Birds, the Ghostbusters opened fire on Tiamat. The Proton Streams had little affect and Tiamat knocked them backwards with a shock wave of energy. Tiamat then transformed into a giant multi-headed dragon and declared she was amiable to skipping over the mind games to something else. The Ghostbusters took aim and crossed 11 streams onto her. Tiamat seemed to be able to contain the energy of the streams then vanished from sight in a white flash. As the Ghostbusters, Dana, and Louis contemplated the end, Blood Rain showered down on them amid Tiamat's laughter.
Over the next couple of weeks, relatively few of the symptoms of Tiamat's presence eased.  Blood Rain continued. Kylie's research did not yield any actionable results. After tearing through everything in Ray's Occult Books for any clue or reference on defeating Tiamat, Kylie could only find advice on general appeasement and the baseline for that was sacrifice. Tiamat reveled in the fact she withstood the Ghostbusters' technology and sensed Gozer's remaining essence was in a dormant and anemic state.  Tiamat continued to toy with the Ghostbusters in order to jilt Gozer back into fighting form.  She was amused by the Ghostbusters and their unpredictability.  Tiamat's presence on the physical plane finally awakened the remnants of Gozer. Awakened and energized, Gozer vowed to correct past mistakes and fulfill his purpose of destruction. He made direct mental contact with the Selector, Ray Stantz. After the Male Terror Bird was extracted from Louis and trapped, Tiamat briefly appeared on one of the lockers behind Egon and Kylie.
As Gozer continued to try to force Ray to submit to his will, Tiamat crossed over from her dimension to Ray's mind. The two gods did battle in various forms. All the while, Tiamat taunted Gozer repeatedly. To cement Gozer's impending defeat, Tiamat manipulated Ray's memories so that he and thus Gozer wouldn't know the Containment Unit's biometric security measures would bar them access due to possession. Gozer would believe he could take back his captured essence and power up enough to defeat Tiamat once and for all.  Once Gozer realized the truth, Tiamat reached into her brother and tore out his consciousness and feasted on his despair. By tossing him in her mouth, Tiamat banished Gozer to another dimension.  Tiamat took complete control of Ray's body and declared the Ghostbusters' only hope of surviving was not boring her. Tiamat then punched the hull of the Containment Unit several times, threatening to cause a rupture and massive explosion. Tiamat was intrigued when Winston addressed her and offered to sacrifice his own life to appease her.  Tiamat accepted.
Tiamat vacated Ray and stopped Winston before he could enter Heaven. Although Winston's sacrifice was crude and without proper ceremony, the real problem she had was Winston happily died the warrior's death to stop her. She wanted Winston to make a sacrifice and live to face the consequences. Tiamat insisted he choose another sacrifice. Winston refused then Tiamat shoved him away. Winston awoke back in his body and raced off to protect Tiyah. All the while, Tiamat appeared to Winston and taunted him. Once Winston entered the hallway leading to Tiyah's apartment unit, Tiamat appeared in a mirror and gave him one last chance to choose a sacrifice. Winston adamantly refused and shot at her. She simply became intangible and let the stream pass through her. Tiamat imitated Winston's voice and called for Tiyah then knocked on her door. When Tiyah answered, Tiamat lifted her up by the throat and was about to kill her when another idea came to mind. Tiamat extracted a portion of Tiyah's soul and erased her life experience with Winston from the world. Tiamat declared this sacrifice was acceptable and bid Winston goodbye... for now then vacated the dimension.
Tiamat's presence alone can disrupt the fabric of its environment and disrupt that environment's rules of science. She can shape shift and so far, has taken on the form of blonde female human and a multi-headed dragon. She can also manipulate physical surroundings like trapping Dana in her own apartment with brick walls, power to speak to the dead, can manifest on mirrored surfaces and even the face of the Moon, make herself intangible, imitate the voice of others, extract souls, and erase memories of an event from an entire planet.
Tiamat revels in the feeling lesser beings get when hope is taken away from them. 
Based on MythologyEdit
According to Babylonian mythology, Tiamat represents chaos and is a primal goddess of the ocean.
- In Ghostbusters Volume 2 Issue #19, three of Tiamat's forms are visually based on Necksa, one of Proteus' forms, and Quetzalcoatl from The Real Ghostbusters.
- On page seven of Teenage Mutant Ninja Turtles/Ghostbusters Issue #3, a picture of Tiamat is on the white board.
- On page one of Teenage Mutant Ninja Turtles/Ghostbusters Issue #4, Tiamat appears once more on the white board.
- On the Ghostbusters: Mass Hysteria back cover, Tiamat is alluded to in the description.
- In Ghostbusters International #5, the effects of Tiamat's memory erasure were felt when Ray couldn't remember why the Ghost Taser was built. 
- On page 14 of Ghostbusters International #10, Winston refers to the erasing of the world's memory of his marriage to Tiyah. 
- Ghostbusters: The Video Game (Realistic Versions)
- Museum of (Super)Natural History Level
- Mentioned in two audio recordings found in "Board of Trustees" area
- Museum of (Super)Natural History Level
- Ongoing Series
- Volume 2
- Ghostbusters Annual 2015
- Alluded to in the Story So Far page. 
- Ghostbusters 101
- ↑ World of Gozer Audio Displays; Board of Trustees area (2009). Ghostbusters: The Video Game (Realistic Versions) - Museum of (Super)Natural History Level (2009) (Video Game). Terminal Reality. Audio Display says: "Few historical records of this period exist and much of what we know about the time period relies on conjecture, legend, and unverified occultist text. Gozer was worshiped by a large Sumerian subculture dedicated to destruction and chaos. The Gozer Cult waged a long protracted war with the followers of Tiamat in the 4th millennium B.C."
- ↑ World of Gozer Audio Displays; Board of Trustees area (2009). Ghostbusters: The Video Game (Realistic Versions) - Museum of (Super)Natural History Level (2009) (Video Game). Terminal Reality. Audio Display says: "The armies of Tiamat defeated the Gozer Cult and the resulting mythology tells of the victorious Tiamat banishing Gozer from this world."
- ↑ Egon Spengler (2014). IDW Comics- "Ghostbusters Volume 2 Issue #16" (2014) (Comic p.4). Future Egon says: "Tiamat is far more powerful than Gozer."
- ↑ Kylie Griffin (2014). IDW Comics- "Ghostbusters Volume 2 Issue #15" (2014) (Comic p.9). Kylie says: "Gozer was bound by rules. He couldn't enter this dimension without followers paving the way, without securing a Keymaster and a Gatekeeper, without having a form chosen for him. But Tiamat doesn't have those rules. She's a goddess of chaos. The only constant has been a feud with Gozer."
- ↑ Kylie Griffin (2014). IDW Comics- "Ghostbusters Volume 2 Issue #15" (2014) (Comic p.8). Kylie says: "Even their followers have been at odds since basically forever."
- ↑ Ray Stantz (2014). IDW Comics- "Ghostbusters Volume 2 Issue #14" (2014) (Comic p.16). Ray says: "One thing that has been consistent: she's the sister of Gozer and they had a real rivalry going on. In fact she - or her followers - were responsible for Gozer's banishment from this dimensional plane to begin with."
- ↑ Tiamat (2013). IDW Comics- "Ghostbusters Volume 2 Issue #11" (2013) (Comic p.21). Tiamat says: "Your people have defeated my brother. They have shaken the very core of the universe. That interests me. It surprises me. And now... I want to play."
- ↑ Tiamat (2014). IDW Comics- "Ghostbusters Volume 2 Issue #19" (2014) (Comic p.9). Tiamat says: "When I first came back to this plane of existence, it was mostly curiosity. The same worms that once worshiped us could suddenly rip holes in the fabric of the universe... and beat you soundly. I wanted to play with them."
- ↑ Ray Stantz (2014). IDW Comics- "Ghostbusters Volume 2 Issue #14" (2014) (Comic p.15). Ray says: "A few weeks back, an employee at the bookstore -- An employee had an extracorporeal experience wherein he claimed to have met the Mesopotamian goddess of chaos."
- ↑ Egon Spengler (2014). IDW Comics- "Ghostbusters Volume 2 Issue #14" (2014) (Comic p.9). Egon says: "But this is clearly the work of a localized entity capable of making changes on the subatomic..."
- ↑ Egon Spengler (2014). IDW Comics- "Ghostbusters Volume 2 Issue #17" (2014) (Comic p.5). Egon says: "Relatively few of the symptoms of Tiamat's presence on this plane of existence have eased."
- ↑ Tiamat (2014). IDW Comics- "Ghostbusters Volume 2 Issue #19" (2014) (Comic p.9). Tiamat says: "But when I got here I felt your presence pathetic. Anemic. I could withstand the power at their disposal. Why couldn't the mighty Destroyer?"
- ↑ Tiamat (2014). IDW Comics- "Ghostbusters Volume 2 Issue #19" (2014) (Comic p.5). Tiamat says: "I thought playing with your toys might get your attention. Well, what attention there was left to get."
- ↑ Tiamat (2014). IDW Comics- "Ghostbusters Volume 2 Issue #19" (2014) (Comic p.13). Tiamat says: "Your reactions are unpredictable. This amuses me. And the fact that you wield power that very nearly ended everything... this impresses me. Well almost."
- ↑ Tiamat (2014). IDW Comics- "Ghostbusters Volume 2 Issue #19" (2014) (Comic p.17). Tiamat says: "Because I made you forget it. It gave my brother a taste of hope."
- ↑ Tiamat (2014). IDW Comics- "Ghostbusters Volume 2 Issue #19" (2014) (Comic p.17). Tiamat says: "Oh, his consciousness will wake in another dimension. Eventually he'll even be able to destroy again. But for the time being, all he'll do is mope."
- ↑ Kylie Griffin (2014). IDW Comics- "Ghostbusters Volume 2 Issue #19" (2014) (Comic p.8). Kylie says: "Like I told you, best I could find was info about appeasement, and that was kind of a dead end as there's more than one school of thought as to what would appease the Sumerian gods. The nuclear option is human sacrifice, which some say none of the old gods can refuse. But there's no consensus, which means the research is basically worthless."
- ↑ Ray Stantz (2014). IDW Comics- "Ghostbusters Volume 2 Issue #19" (2014) (Comic p.4). Ray says: "Two Class Seven Entities. Two of them. In my mind."
- ↑ Tiamat (2014). IDW Comics- "Ghostbusters Volume 2 Issue #15" (2014) (Comic p.5). Tiamat says: "I do truly love the feeling you people get when hope is snatched away. It's like nothing else."
- ↑ Ray Stantz (2016). IDW Comics- "Ghostbusters International #5" (2016) (Comic p.7). Ray Stantz says: "It's a personal defense prototype Egon and I put together -- though I'm having trouble remembering why. Think of it as a taser but for ghosts! It's purely defensive but..."
- ↑ Winston Zeddemore (2016). IDW Comics- "Ghostbusters International #10" (2016) (Comic p.14). Winston Zeddemore says: "I used to be married, you know? And no one remembers. Our whole relationship was erased by an "event"."
- ↑ Narrator (2015). IDW Comics- "Ghostbusters Annual 2015" (2015) (Comic's Story So Far). Narrator says: "They've faced the sister of Gozer -- the Sumerian deity of chaos."
- ↑ Ghostbusters 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #3" (2017) (Comic p.24). Ghostbusters 101 Class Notes reads: "Seen during an encounter with class-7 entity Tiamat, we were unable to get a sample."