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Ernesto Watertide didn't meet such a swift end as he and the other guy had been colluding with the Vampires and were probably the ones that allowed them to remain here in the underground doing all sorts of experiments with the chimera and slaves.
However, as much as I wanted to kill him, I instead possessed him using Possession, forcefully taking over his body and pretending to be him. Now that I had a lot of Mana, this was more than possible.
I feel like I've only used it very little, but a ghost's unique ability to possess people is very useful on these occasions! Although strong enough beings won't easily let me possess them, if they're softened enough (beaten down) or they're too weak to resist, I can easily take over their bodies and even learn any info they could have inside their brains.
Check "The Protagnist's Sister Is Actually the Strongest"!