Yeah, you can perma die only if killed in the castle (like homeplane). However if killed in other plane then your usual banned century or so entering it (as go by classics in regards being killed in prime and then banished from entering it there with real body). Most demon princes (and archdevils etc) worry fighting in own plane (hell, abyss etc). Your main worry is not to die in the castle as you reform in there.<br/><br/>Now as point if you do pick mirror and everyone bonds into it, it basically allows create 'plane inside plane' sort space. Own plane where your literally a god and can reform there instead. Entrance to your own domain can be closed off or smooth transition (instead entering via mirror in castle it can be archway, door, whirlpool or flower). But same sacrifice/limit remains; no reflection nor able take things out from mirror dimension (nor things changed in mirror dimension; so can kidnap princess and transform into ultimate man slut but man slut cannot exit the dimension).<br/><br/>Ofc can decide entry is easy but exit is hard. Overall mirror artifact would give neat toy/means build own lair without limits.