I have also heard that this has been authoritatively defined by no less of an authority than Joseph of Aramathea:
To the north there lies a cave – the cave of Caerbannog –
wherein, carved in mystic runes upon the very living rock, the
last words of Ulfin Bedweer of Regett proclaim the last
resting place of the most Holy Grail.
The writing is proported to reveal that the Castle Aggh is the final resting place of the Grail, but I have never been there myself, as I know very little about unladen swallows (after all, I am not a king).
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