As the sign says:
In the late 1700's, Kamehameha 1, from the island of Hawaii sought to unite all the Hawaiian Islands under one rule. The Battle for Oahu began with the arrival of his forces at Waikiki in 1795.
Oahu had been defeated by Maui forces a decade earlier and Maui's Chief Kalanikupule now led the forces on Oahu. After many hard-fought battles, he was driven up Nu'uanu Valley to this location. Both sides fought with Hawaiian spears and Western firearms but Kamehameha's cannon gave them the winning advantage.
The battle, called Kaleleka'anae (leaping of the 'anae fish), refers to the men forced off the cliff during the battle. An estimated 400 warriors died in this battle. With Kamehameha's victory on Oahu and the signing of an agreement with Kaui, he became the first King of the Hawaiian Islands.

When the road to the outlook was under construction more than 500 skulls were found at the bottom of the cliff where the warriors landed. WOW!