Comet Tiamat

Comet Tiamat is a comet with great significance in the movie. In the opening of the movie, a comet is seen falling through layers of cloud in the sky. It appears to fall onto the town of Itomori, where people are having a festival.

A comet is an icy, small Solar System body falling. When passing close to the Sun, it warms up and begins to release gases, a process called out-gassing, which gives it a stream "tail".