Tiamat & Bahamut

Tiamat and Bahamut are two of the most powerful and iconic deities of dragon belief. Tiamat, the five-headed, chromatic dragon goddess, embodies chaos, destruction, and greed. She is the progenitor of evil dragon types and one of the main antagonists of many campaigns within the multiverse of Dungeons & Dragons.
Bahamut, on the other hand, is the god of justice, wisdom, and honor and is depicted as a silver-white platinum dragon. He is the patron of good, usually metallic dragons and a symbol of noble behavior and divine assistance.
Both dragons have their roots in real mythology. Tiamat originally comes from the Babylonian creation myth called Enûma Elish. She is portrayed as a primordial ocean, a chaotic sea monster, or a divine progenitor who fights against the younger gods.
According to the myth, the world was created from her shattered body. Bahamut originates from an Islamic legend in which he is a giant, primordial fish or whale that carries the world on his back.
Thus, in Dungeons & Dragons, the cosmic fish became a noble dragon patron, while the sea goddess turned into a terrifying symbol of evil. Nevertheless, the original ideas of creation, order, and chaos can still be found in the implementations in D&D.