Long before the twelve gods of Olympus came to power, people worshipped and were ruled by the sons of Gaia (Earth) and Ouranos (Heaven). These were the giants, or the Titans. In a battle that shook the very foundation of the universe, these giants struggled against the Olympian gods, led by Zeus the son of the Titan Chronos (Time).
The Titans were eventually defeated and after the battle, the giant known as Polybotes fled to Kos to seek refuge. The god of the sea, Poseidon, had a particular hatred for Polybotes and pursued him there. When Poseidon found Polybotes he broke off a part of Kos Island and hurled it at him, crushing the giant and killing him. This, according to myth is how the isle of Nisyros was formed. (Nisyros was in fact part of Kos but was separated by an earthquake. The exact time when this occurred is unknown.)