Comments posted to our Dark Souls 3 Wiki
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I somehow finished Grand Archives in one play through including the boss. Without returning the cinders, I gave the crystal scroll to Orbeck and went for second search in Grand Archives. When I returned, I found that Orbeck has been gone and then I successfully looted his corpse. So I wonder 'If you do not purchase all his spells prior to this bossfight he will remain in the Firelink Shrine and will not die.' may not be true.
My experience confirms your theory, you do not have to purchase all spells prior to the bossfight.
Damn it, I already killed the abyss watchers :( NG+ will be then
He said something different to me upon killing me, i am in ng+ and wanted to test some of my weapons so i attacked him, after he killed me he said something like "Gods, what have i done?"
To make him available for the princes boss fight I had to buy all sorceries again in NG+ though I completed his questline in the first playthrough.
I have 10 Int and he still woudn't recruit me. More than 10 is required.
You need to talk to him again. At least that worked for me.
I made a new character and went straight for Orbeck of Vinheim, except when I got to the location where he should have been, only his ashes were there.
I did transfer some items from my other character to the new one, but other than that it was just a standard run to his location. Any idea why his ashes were there, and not him?
I beat the entire game except gael. Every boss and opt. Went back there to give a friend a walkthrough and he was there. This was my game. I was talking the guy through not showing him. So he never disappeared even though i have killed everyone.