Winter Cave is an event island added in GPO's Christmas 2021 update located in the Second Sea. Boats or other forms of transportation are unable to travel to the island as it is only accessed via Santa's Sleigh teleporter in Winter Wonderland.
In order to get to the Winter Cave, you must drop 15 Gingerbread Giftboxes in Santa's Sleigh. Then the teleporter will open, allowing you to teleport there. It has to be noted that once you teleport to the Winter Cave, you cannot leave until defeating Krampus or dying.
The cave is surrounded by water which can kill fruit users and sharks will spawn. You can also spawn your boat and Krakens/Sea Beasts. Krakens and Sea Beasts do not help in fighting Krampus as most moves besides the Sea Beast's water stream does not usually hit Krampus.
There is only one quest on the island and the quest giver is "Santa (HURT)" in which he tasks you with fighting Krampus, the Ravager. The quest level requirement is 325+ and it is HIGHLY recommended that you bring a group of 6+ people to fight Krampus as it is a hard boss.
NPCS: Krampus, the Ravager (BOSS)