The second time you meet Jagon, I can not kill him but if you can atack him enough (so his HP goes down), on that moment him runs away! You shall meet him again a third time and also a fourth time! I think the fourth time I can kill him and loot his necklage for a quest-reward
Only savegames IN this Temple? >> I hope you have enough healingpotions or a healingspell with you?
The posion is strong and there are other "traps" in these Temple of Doom, so be careful!
And a warm welcome to the forum
