An Ultra Wormhole in Pokémon GO is a special portal that appears above Gyms and works like a five‑star raid egg, but it always spawns an Ultra Beast instead of a regular Legendary boss.

What an Ultra Wormhole Is

  • It’s a swirling, portal‑like icon that appears over a Gym where you’d normally see a raid egg.
  • When the countdown ends, the Ultra Wormhole “opens” and an Ultra Beast (like Nihilego, Buzzwole, Xurkitree, etc.) appears as the raid boss.
  • In gameplay terms, you treat it just like a 5★ raid: join the raid, battle the boss, and then try to catch it.

Think of it as a themed 5★ raid egg that guarantees an Ultra Beast instead of a standard Legendary.

How It Works in Raids

  • Difficulty is similar to other five‑star raids; you’ll usually need several trainers unless everyone is very strong and well‑prepared.
  • During certain events, Premier Balls can be replaced with Beast Balls, which have a higher catch rate for Ultra Beasts.
  • Which Ultra Beast appears is controlled by the current raid schedule and events, just like normal raid rotations.

Why Ultra Wormholes Matter

  • Ultra Beasts are among the strongest raid attackers in the game; for example, Nihilego is a top Poison‑type and Xurkitree is one of the best Electric‑type attackers.
  • Because of their power and limited availability, it’s usually worth going out of your way to do Ultra Wormhole raids when they pop up nearby.

Quick FAQ Style Recap

  • “Is it dangerous or super complicated?”
    No—mechanically it’s just a special five‑star raid with unique visuals and Ultra Beasts as bosses.
  • “How do I know one is nearby?”
    Check the Nearby raids tab; a portal‑like symbol indicates an Ultra Wormhole raid is scheduled at a Gym.

TL;DR: In Pokémon GO, an Ultra Wormhole is a fancy 5★ raid “egg” that guarantees an Ultra Beast raid boss when it opens, making it a prime target if you want powerful end‑game attackers.

Information gathered from public forums or data available on the internet and portrayed here.