World Boss is an upcoming game mode that was announced on Episode 73 of the Podcast.
You can also read about the upcoming World Boss mode in this blog post.
Challenges are presented once the player beats The Swamped Ruins of Koruzuran the Crocodile on Hard Difficulty. They are offered up by Jables, the Jester, who dances around town and sings about Challenges all day.

“The Fast and the Furriest” is a Challenge Game Mode where the player is pitted against 8 waves of Puke Rats at all rarities, including a Puke Rat Boss.

“Oops, All Glubs!” is a Challenge Game Mode where the player is pitted against 8 waves of Glubs at all rarities, including a Glub Boss.

“Mo Problems” is a Challenge Game Mode where the player is pitted against 8 waves of Mo at all rarities, including a Mo Boss.
The Swamped Ruins of Koruzuran the Crocodile are accessible through the Breach, which opens once the player defeats the Sewer Troll in Dungeon 10. It is commonly referred to as “the Breach” by most players.
The Ruins are in a murky swamp filled with menacing mobs ready to attack the character.
The following Mobs can be found in Tier 1 Dungeons.