Title: QuantumTale

In this AU, the Underground is not a traditional subterranean realm but exists within the fabric of quantum possibilities. The characters from Undertale find themselves in a world where reality is fluid, and the laws of physics are subject to constant flux.

Setting:

The QuantumTale universe is a surreal and ever-changing landscape where timelines intersect and parallel realities coexist. The characters navigate through shifting dimensions, encountering alternate versions of themselves and their surroundings.

Premise:

An anomaly disrupts the stability of the timelines, causing rifts between different quantum states. Frisk, the protagonist, must traverse through these quantum fluctuations, interacting with alternate versions of the characters and facing challenges that test the very nature of reality.

Characters:

1. QuantaFrisk - The Quantum Traveler: Similar to Frisk, but with the ability to manipulate quantum probabilities. QuantaFrisk's journey involves navigating through various quantum states and repairing the fabric of reality.

2. QuantumSans - The Observer: Sans possesses an awareness of the quantum fluctuations and often breaks the fourth wall, acknowledging the player's impact on the timelines. His jokes and puns take on a meta-narrative, reflecting the fluidity of the QuantumTale universe.

3. QuantumPapyrus - The Multi-Dimensional Enthusiast: Papyrus is fascinated by the idea of alternate realities. He enthusiastically explores different versions of himself, offering humorous commentary on the quirks and differences between quantum states.

4. QuantumUndyne - The Protector of Constants: Undyne is determined to maintain stability in the quantum realm and serves as a guardian against anomalies that could threaten the existence of the characters and their world.

Locations:

1. QuantumHub: A central point where timelines converge. QuantaFrisk must make decisions that ripple across quantum realities, affecting the characters and the overall narrative.

2. ParadoxPlains: A landscape where contradictory events coexist. QuantaFrisk faces puzzles and challenges that require them to embrace the paradoxical nature of the QuantumTale universe.

3. ProbabilityPaths: A network of interconnected paths that represent different potential outcomes. QuantaFrisk must navigate through these paths, making choices that determine the progression of the story.

4. InfinityCore: The heart of QuantumTale, where the quantum anomaly originated. QuantaFrisk confronts the source of the disturbance, making choices that will either restore balance or plunge the universe into chaos.

Storyline:

QuantaFrisk's journey in QuantumTale involves unraveling the mysteries of the quantum fluctuations, interacting with alternate versions of the characters, and making decisions that shape the very fabric of their reality. The narrative explores themes of choice, consequence, and the interconnectedness of all possibilities within the QuantumTale universe.