Hey was looking into some solutions as well and stumbled onto this.

My group around lv10 is fighting a demilich along with summonable gargoyle minions so 1 shotting a PCs full health at 0 isn't happening.

and so even though one of my players had the periap for a while and it has saved them a few times, it really only 'thwarted' me with the demilich howl (on a successful save you fear the demilich on a fail instant 0 hp) and while I'm not an adversarial DM by any means and bend over backwards to have my mobs fumble when theyre in the corner (not that I wont kill my players but doing things like stabbing while down is a big no no unless that mob has NO other options)

so I've been looking for a way to counter the player but just realized...... ANTI-MAGIC FIELD! Since it ends the magic of magic items too! The demilich has it as a lair action so that works but it only lasts a turn so at most I can get him to fail a single save or so.. which i think would still offer that fear factor i want to instill even if it doesnt kill him (although if I want, I can have the demilich instruct one of the minions to attack mid turn since the demilich only inflicts saving throws and no attack rolls (and I leave KO characters out of AoEs, it would be unfair to change it now especially since I'm not really out to get anyone in particular  )

If you have a smart or prepared boss I would like to suggest to use an anti magic field (or beholder) to counter it and as a spell it should last longer too

Hey was looking into some solutions as well and stumbled onto this.

My group around lv10 is fighting a demilich along with summonable gargoyle minions so 1 shotting a PCs full health at 0 isn't happening.

Even though one of my players had the periapt for a while and it has saved them a few times, it really only 'thwarted' me with the demilich howl (on a successful save you fear the demilich on a fail instant 0 hp) and while I'm not an adversarial DM by any means and bend over backwards to have my mobs fumble when theyre in the corner (not that I wont kill my players but doing things like stabbing while down is a big no no unless that mob has NO other options)

I've been looking for a way to counter the player but just realized this: ANTI-MAGIC FIELD! It suppresses the effects of magic items.

The demilich has it as a lair action so that works but it only lasts a turn so at most I can get him to fail a single save or so. I think it would still offer that fear factor I want to instill even if it doesn't kill him (although if I want, I can have the demilich instruct one of the minions to attack mid turn since the demilich only inflicts saving throws and no attack rolls. (I leave KO characters out of AoEs, it would be unfair to change it now especially since I'm not really out to get anyone in particular).

If you have a smart or prepared boss (or a beholder), it can use an anti magic field to counter the periapt.