The Tale of the Obsidian Key (Gilgamesh X Artoria)

News reaches Uruk that a devastating wave of corruption, manifesting as shadowy beasts and strange mental anguish, is spreading from a distant, ancient ruin known as the "Sundered Sea." Artoria, seeing her distant coastal villages afflicted, sails to Uruk to propose a joint mission: The source is an artifact too dangerous for one kingdom, perhaps the key to a lost Gate of the Gods that Gilgamesh's records hold.
Gilgamesh is, as usual, arrogant but intrigued by the challenge and Artoria's audacity. Enkidu, sensing the danger to both their kingdoms, ensures the alliance proceeds. The two kings and one advisor journey together, with Artoria focused on the duty, Gilgamesh focused on reclaiming the "treasure" (the Obsidian Key), and Enkidu focused on the well-being of his friends and the world.