Combining Don't Dig Up and Celebrationmk10 should have the Ash Trees in the Underworld replaced with gem trees. Also, the lava in the Underworld is replaced with Shimmer, and the Ruined Houses are painted in fun colors. The center of the Underworld, where the Terrarian spawns, is considered to be the Aether biome. However, the Shimmer makes the Underworld
even more dangerous, as the Shimmering debuff is no longer removed by the Underworld.
If a Terrarian manages to die by falling out of the world, a unique party-themed death message will appear, as this death is a new death exclusive to this world. The Terrarian can also die by falling out of the world without Shimmering in this seed combination, so don't think you're safe even if you removed it all!
"[Terrarian] blacked out!" OR "[Terrarian] had a bit too much to drink!"
Party-themed messages will also occur if the Terrarian manages to die by leaving the world at the top border.
"[Terrarian] is as high as a kite!" OR "[Terrarian]''s ride arrived!"
Similar to the Zenith seed, the Glowing Mushroom biome above the Underworld is also painted in rainbow colors here (and so is the Jungle Temple). The Guide is replaced by the Party Girl, and all NPCs are their shimmered variants by default. NPC happiness is also reenabled. A Party will also start upon spawn, and the Princess and a Black Town Cat will spawn as well (black cats have historically been seen as demonic, a fitting underworld replacement for the Angora Town Rabbit of Celebrationmk10).
As Queen Slime does have a similar appearance to Shimmer Slimes, Queen Slime will also spawn in the Underworld's Aether biome unprovoked sometime after entering Hardmode, like the Eye of Cthulu for pre-hardmode.
While generating, this world has the "Aether" theme play, and all generation descriptions are violet-colored, similar to the color of the Shimmer.
This world has a bit of a Dionysian feel, as it is essentially a party in the underworld surrounded by what looks like lakes of magic champagne, while the world above on the other side of this hellhole is a deadly dystopian disaster zone. Forget your worries and drink your way to your deaths!