See excerpt from Solec v Salyeris below.

With a flick of his tail, a manticore looses a volley of iron tail spikes at you.
A manticore's iron tail spike grazes you.
A manticore's iron tail spike strikes you in the chest, leaving a lasting wound.
A manticore's iron tail spike injures you.
A manticore's iron tail spike strikes you in the chest, leaving a lasting wound.
A manticore's iron tail spike maims you!
A manticore's iron tail spike strikes you in the legs, leaving a lasting wound.
A manticore's iron tail spike MASSACRES you!
A manticore's iron tail spike >>> ANNIHILATES <<< you!
That really did HURT!
A manticore's iron tail spike does UNSPEAKABLE things to you!

The damage used to do a straight multiplier with each spike (I believe tripled the previous), up to 6, or maybe 7 hits (don't recall and I did not survive the 6th hit).
based on the damage verbs, I think it was a 3 hp graze on first hit. Essentially multiply the initial damage by 364, that's what it would do. So even a 1 hp strike was a net 364, which is insane.
In this one, 3 9 27 81 243 729.
1092 damage, through anthem and resist. A seventh hit would have done over 2000, but I do not recall if it got a seventh hit.
I'm also not sure if it bypassed all dam redux or not, but if it didn't, on the average npc with no dam redux, you can figure out that it was one shotting almost all of them.

So it wasn't insta kill, but the damage was essentially so much that it could not be survived.

Battlecharmed did report the issue though, so I believe I was the only one he ever used it on in PK when it did this, because it was so obviously broken. I think he did admit using it to in non PK fights though. You'd have to check with him to see just how OP it initially was.