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Gustav apparently had slept for a whole week after the incident and decided to take a bath with the girls after finally waking up to feel refreshed. However, through the bath, various lewd things ended happening, and the vicious pair of fusions drained him dry of his energy right after he woke up…
Nonetheless, after that, he was given a big feast, which included a lot of preparations using the meat of this monstrous being, which was highly nutritious when cooked by the flames of the infernal demons that Natasha could conjure.
The meat was still alive even after the creature was defeated, but they had to burn it greatly for it to finally die. Nonetheless, after that, Gustav acquired a great amount of power by simply eating these delicious dishes. It seems that the meat of an otherworldly interdimensional being was quite nutritional.
Check "The Protagnist's Sister Is Actually the Strongest"!