Wraith can reduce max hp tally by damage done, death if max hp goes to 0. I think the Wiz-5 had 22 hp and it did 22 damage & she failed CON save, so insta-kill. 22 is 1 point over its average damage!
Roll20
It wasn't a TPK; after the wraith raised her as a Spectre and the Cleric failed to turn it, the two remaining PCs fled in terror.
I saw another PC perma-killed by a few Shadows draining his STR to 0. Shadow is only Challenge 1/2 -
Shadow - so vs a high level group you can 'legally' use a pretty ridiculous number.
For that matter the Banshee is CR 4 and can insta-kill
Banshee
And of course there's the CR 2 intellect devourer...
https://media.wizards.com/2014/downloads/dnd/MM_IntellectDevourer.pdf
Well, not knowing the rest of the encounter, a single Wraith got lucky by smacking a Wiz-5 with 22 hp (which, FWIW means
NO CON bonus and/or crappy HD rolls) for a critical hit and the PC failed the CON save (again, with no or little bonus). So, while the 22 damage is basically average for the critical hit, the odds are pretty rare (about 1 in 30). I guess the PC also had no inspiration to reroll the failed save or anything?
Then the cleric failed to turn the Specter? Again the odds are against that as well. Specter's have no WIS mod or save, so it was a straight d20 roll against the Turn DC, which at 5th level should be about 14 or so. That is only about a 1 in 3 chance to make the save. Not bad, but 2-1 against.
So, not knowing the rest of the party (other than a cleric), even with all that being the case, the three remaining PCs would have a "hard" encounter against the Wraith and Specter combined instead of the "easy" it would have been with all four against the Wraith alone. Unless another unlucky crit came up, odds are the remaining three should prevail (dependent on other factors such as resources (spells, feature uses, etc.) remaining and hp and such).
Against the same 4 5th-level characters, using 15 or more shadows would be a deadly encounter, but look at it a bit more closely...
The cleric alone can
destroy (not just turn) shadows. Even with only one channel divinity, that 30 ft. means you would likely get the bulk of them and 2 out of 3 on average would be destroyed by failing the DC 14 WIS save. So, suddenly those 15 are probably reduced to 5-10 (at most!). And 5th-level is hardly "high level" for characters.
The Banshee and Intellect Devourer can both be tough foes, certainly, but so much there depends on the encounter set-up and party composition, etc. and basically how much the DM just wants to kill-off PCs.
Finally, from the beginning of this thread, the point has never been that it is hard to kill PCs (any DM can toss whatever they want to make a TPK-not hard at all), but that short of a TPK, keeping people dead isn't as easy. Short of really long-shot unlucky rolls or tossing hordes of such creatures against a party, but don't solve the issue as I see it.