The Wither

The Wither was a boss summoned twice in the Squid Island series. The trio first summoned the Wither in New Squid Island, and as soon as it was spawned, it ravaged the landscape around it. Crainer and Slogo were mostly fighting the beast as Jelly watched from afar. It was an incredibly tough battle, but soon Slogo prevailed and collected the Nether Star which was used to create the Squid City Beacon, later taken by Slogo and put in the museum in Slogo Land controversially and finally being placed as the new beam for the flag of the United States of Squid Island by force of the Jelly administration before the destruction of Squid Island.
The second and latest time they spawned the Wither was on Crainer’s Pirate Island. Crainer got several wither skeleton skulls to make pirate banners given his pirate theme, but three were given to Slogo - albiet hesitantly - and Jelly helped Slogo spawn the Wither unplanned as Crainer was busy. The Wither exploded the lobby of the Pirate Watchtower, before ravaging several parts of the island and killing Crainer and sending him back to the spawn point - confirming the audience analysis that season two is set on a different world than season one. Slogo and Jelly lured the Wither over their boat and paddled to Dog Island adjacent to Jungle Island to prevent further damage to Pirate Island, where the trio successfully killed the Wither again.