I *think* this is how it happened: I had a brand new Arcane Blade fighting the penultimate elite in Trollmire (the one before the last area, I forget whether that's Bill or Prox). I was on the exit to the previous area and taking heavy damage, so I retreated to rest. When I changed areas I got the Transmogrification Box dialog. I moved some stuff to my normal inventory and the troll got in another hit, killing me. Eidolon Plane, rest up, return. For some reason it dropped me on the world map. I don't think I hit "return me somewhere else", but it's possible. I noticed that I had an icon showing the Eidolon Protection effect. Went back into Trollmire, but there was only one area. No exit except to the worldmap. Went on to other dungeons, wasn't taking damage. Yup, Eidolon Protection still active. Start messing around. Found out that drowning is considered to be self-inflicted and you're not protected. Die. Come back. The Eidolon Protection icon is gone, but the effect is still active. Everything still does 0 damage to me.
Bug, or did I accidentally activate an exploration mode? But if that's the case, why is the game giving me achievements?
So now I'm just exploring all the places I never managed to get to before. I might end up resetting the achievements afterward, or I might just play around with the races/classes I'm busy unlocking. It's not cheating if it's a bug, right?