he might got it from transform .
sorry i dunno how to deleted ;-;
Probably invulnerability from Sterea Trofh (Druid C2).
It had the icon of certain monsters that go into temporary ‘immortal’ status, so the closest thing I can think of is that he got it from a specific monster by using Shapeshift/Transform as a Druid C2.
If no one there was Druid C2, then next guess would be an exploit.
druid has a transform ability, some monsters have invincibilty skill. its not exactly a new buff, but its a exclusive buff, only to monsters and druids