Tiamat is the large alien ship (or being) that had been lying dormat in the Oort Cloud for thousands of years until she was awakened by the ASE at the end of Conduit 2 because all of the Progenitors have been killed and their essences absorbed.
In Sumerian Mythology, Tiamat is the goddess of chaos, destruction and the ocean. Tiamat is also believed to be a planet in conspiracy lore.
A document in Siberia reveals that Tiamat is a large sentient computer that contains near limitless knowledge of the universe.
In one point in Conduit 2's storyline, Andromeda refers to the ASE as the "Eye of Tiamat"
Tiamat Dossier Trust Archive File
Tiamat is a dark and angry beast, filled with both unfathomable rage and infinite patience. Unaffected by association with humans, Tiamat's mind works in a totally alien fashion. Tiamat functions as a living computer that controls the [CLASSIFIED] she calls home. Her miles of tentacles/eyestalks wind throghout the entire place, putting her in perfect contact with everyone in the [CLASSIFIED]. Armed with total information at all times, she uses it to ensure that the entire place-and everyone in it-works together in cold and efficient harmony.