Anime Crossover: The Demon Lord of Black Sun Castle

After transmigrating to another world, he opened his eyes to find himself in a medieval castle. A black sun blazed overhead, scorching the sky until it cracked, while the grasslands beyond the walls stretched to blood-red ridges. Two rows of maids bowed respectfully and called him Master. Before he could ask where he was, the System flashed a warning: the castle's defensive shield had only 3% energy remaining, and after dark, monsters from the black sun would crawl out to devour people. He turned to the maids. “Do you have any weapons?” They all drew long knives from beneath their skirts in perfect unison.