Onigashima

Onigashima is an island located somewhere in the New World. Kaido of the Yonko resides here.

Geography
Onigashima appears to be a Winter Island, with snow constantly falling. The landscape is dotted with bonsai-like trees.

Kaido's Residence
Kaido resides in a giant cave that resembles a skull with long horns on top. This cave occupies a considerable portion of the island. The cave has a cell which is currently occupied by Eustass Kid.

Dressrosa Arc
X Drake and some other Beasts Pirates were outside Kaido's lair on Onigashima when they received news of Joker's defeat, which they tried shouting out to Kaido.

Zou Arc
Sometime later, Kaido returned to his lair and received a call from Scratchmen Apoo, telling him they lost contact with Jack. Realizing his plans to take back Doflamingo had failed, Kaido drank a lot and suffered from intense mood swings, including a rage that caused him to hit some of his subordinates through the roof of his lair.

Reverie Arc
Kaido received a call from Big Mom, who told him she would be coming to his territory to kill Luffy despite his adamant refusal.

Trivia

 * The island shares its name with an island in the Japanese folktale of Momotarō, where the titular hero and his animal retainers went to defeat the residing demons that were troubling people, similar to how the Ninja-Pirate-Mink-Samurai Alliance plan to take down Kaido who was oppressing the people of Wano Country.