"Metis was both a threat to Zeus and an indispensable aid (Brown 1952:133):

"Zeus lay with Metis but immediately feared the consequences. It had been prophesied that Metis would bear extremely powerful children: the first, Athena and the second, a son more powerful than Zeus himself, who would eventually overthrow Zeus.
In order to forestall these dire consequences, Zeus tricked her into turning herself into a fly and promptly swallowed her. He was too late: Metis had already conceived a child. In time she began making a helmet and robe for her fetal daughter. The hammering as she made the helmet caused Zeus great pain, and Hephaestus either clove Zeus's head with an axe,[7] or hit it with a hammer at the river Triton, giving rise to Athena's birth. Athena leaped from Zeus's head, fully grown, armed, and armored, and Zeus was none the worse for the experience."