I am not sure if I am the only one but the ability "Piece of Hell" is misleading to me. I think this comes from my experience in other card games, primarily Magic.
Piece of Hell reads
When on the field, this unit cannot be targeted by Grimoires and Soul Skills.
My issue is the word targeted. I think the more appropriate word would be "affected".
Target and targeted, to me, are when an effect directly "targets" something. Ex: Fire Arrow, Assassin soul skill
"Target" is not something like Shear Winds, which affects all units without targeting any.
I hope I am not causing confusion, it is just the wording was very misleading to me in what I thought the unit could theoretically do and what it can do in practice.