"Undertale Yellow" is a fan-made game based on the world of "Undertale," developed by Team Undertale Yellow. While "Undertale Yellow" is not an official release, it's inspired by the same universe and characters created by Toby Fox. Here's a fictional story concept for "Undertale Yellow":

In the world of "Undertale Yellow," players are introduced to a new protagonist, a young monster named Sunny. Sunny lives in the outskirts of the Underground, a place where monsters have been banished after a war with humans long ago. Despite the peaceful coexistence established by the monsters, tensions still linger between the two races.

One day, while exploring the caverns near their home, Sunny stumbles upon a mysterious artifact known as the Golden Heart. This artifact is said to possess immense power and is rumored to have the ability to break the barrier between the Underground and the surface world.

Fascinated by the artifact's potential, Sunny decides to embark on a journey to uncover the truth behind the Golden Heart and its connection to the barrier. Along the way, they encounter a colorful cast of characters, including other monsters seeking freedom, as well as human explorers who have ventured into the depths of the Underground.

As Sunny delves deeper into the mysteries of the Golden Heart, they must navigate through challenging puzzles, engage in turn-based battles with monsters and humans alike, and make crucial decisions that shape the outcome of their journey. Along the way, they learn valuable lessons about empathy, compassion, and the power of friendship.

However, not everyone in the Underground shares Sunny's peaceful intentions. A group of radical monsters, led by a charismatic leader named Goldius, seeks to use the power of the Golden Heart to wage war against humanity and reclaim their dominance on the surface.

Sunny must confront Goldius and their followers in a climactic battle to prevent a catastrophic war that could destroy both worlds. With the help of their friends and allies, Sunny ultimately prevails, using the power of compassion and understanding to convince Goldius to abandon their destructive plans.

In the end, Sunny's journey culminates in a bittersweet victory as they come to terms with the complexities of their world and the challenges that lie ahead. As the sun sets on the Underground, Sunny and their friends look towards a future filled with hope and possibility, knowing that their actions have helped bridge the divide between monsters and humans.