The alarm sounded loudly, announcing "Start self-destruct mode, proceed to release all specimens." Thousands of monsters escaped the experimental cage and rushed to Kosu and Sakura.

Kosu and Sakura's team quickly realized the gravity of the situation and began to fight their way out. Masaru used his massive axe to create a path through the horde of creatures, while Akira provided cover fire with her bow. Shiori used her restorative magic to heal any injuries sustained by the team.

As the team fought their way out of the underground facility, Sakura turned to Kosu with a determined expression on her face.

"Kosu, we need to free the dungeon's heart from Black Sun's grasp. But we can't just destroy it, it holds too much power. We need to release it back into the dungeon, to let it become magical particles and reside elsewhere," Sakura said.

Kosu nodded, understanding the gravity of the situation. He unsheathed his sword and focused his energy on the dungeon's heart. With a swift strike, he shattered the crystal into millions of tiny pieces. The particles glowed brightly before disappearing into the air, flowing back into the dungeon.

As the last of the particles dissipated, the ground beneath them began to rumble. Alarms blared, signaling the self-destruct sequence had been initiated. The team sprinted towards the exit, fighting their way through hordes of monsters.

Despite the danger, Sakura remained calm and focused, leading the team with precision and strategy. Akira's sharp eyes and quick reflexes took down enemies from afar with deadly accuracy. Masaru's massive axe cleaved through the toughest foes with ease, while Shiori's powerful restorative magic kept the team going strong.

Kosu and Sakura fought side by side, their swords working in tandem as they took down Black Sun soldiers left and right. Their teamwork was flawless, each one anticipating the other's moves and covering for each other's weaknesses.

As they made their way through the facility, the sound of alarms blaring and explosions echoed throughout the halls. Kosu and Sakura fought back to back, their swords flashing in the dim light as they took down wave after wave of Black Sun soldiers.

Finally, they reached the exit, but it was blocked by a giant monster that had been created in the facility. The monster was heavily armed and seemed almost invincible. 

Kosu knew he had to act fast. He took a deep breath, closed his eyes, and focused his mind. He concentrated all his energy on his sword and, in a split second, unleashed his Mind Slash technique.

With a swift motion, he sliced through the air, and a burst of energy shot out from his sword, obliterating the monsters that stood in their way. Sakura and the rest of the team watched in awe as the monsters fell one after the other.

Kosu's Mind Slash was a powerful technique that he rarely used. It allowed him to channel his mental energy into a powerful sword strike that could cut through almost anything. But it also took a toll on his body, and he could only use it sparingly.

As they made their way towards the exit, Kosu could feel his strength waning. He knew he couldn't use the Mind Slash again, and he hoped that they could make it out of the facility before encountering any more monsters.

Sakura and the rest of the team followed closely behind Kosu, ready to defend themselves if necessary. But to their surprise, they encountered no more monsters as they made their way out of the facility.

As the team finally made it out of the underground facility, they felt a sudden tremor beneath their feet. A moment later, a deafening explosion shook the entire forest, sending shockwaves rippling through the trees.

The group watched in horror as flames and smoke billowed up into the sky from the direction of the Black Sun's lab. They knew that the self-destruct mechanism had been triggered, and the facility was now a pile of burning rubble.

Sakura breathed a sigh of relief, knowing that they had made it out just in time. She turned to Kosu and thanked him for his quick thinking and skillful use of his Mind Slash technique, which had helped them get through the horde of monsters blocking their path.

As the team walked away from the scene of the explosion, they couldn't help but feel a sense of triumph mixed with sadness. They had accomplished their mission and put an end to the Black Sun's evil plans, but at a great cost.

Kosu and Sakura exchanged a look of mutual understanding, knowing that they had been through something life-changing together. They knew that they had formed a bond that could never be broken, and they would always remember this moment as the time they fought side by side and emerged victorious.