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Frank had deposited the souls of his parent inside of a special sanctuary within his divine realm, it was named spring of soul healing, a place that had been naturally formed alongside many other divine materials, this spring counted as a Soul Attribute Divine Material, and held soul-healing properties.
He had left them rest there as they were put to sleep and relax. He wanted them to rest and not be conscious so they wouldn't lose more thoughts and they wouldn't fall into disparity again.
If they were left too much as souls, they would begin forgetting things and feel weakened at times, which wasn't something Frank wanted to occur anymore, which had happened when he had left them free before… He had grown concerned that they might develop Alzheimer's or even Dementia, so he decided to put them into a coma until their reincarnation…
It was a hard decision, but it had to be done.
Check "The Protagnist's Sister Is Actually the Strongest"!