Below is a walkthrough for Noelle, Seeker of Knowledge (I), a Side Quest in Octopath Traveler. You will need either Ophilia or Primrose to complete this quest.
You can start this quest at Cobbleston after completing Olberic's Chapter 1 quest, and by speaking to Noelle.
"I seek traces of the legendary King Beowulf the Great. If you happen to come across some ancient plinth, or carved granite, pray tell me at once."
Guide / Allure Noelle.Interact with the plinth inside Untouched Sanctum.
This quest will require you to explore a dungeon - it is the Untouched Sanctum, located in the North Cobbleston Gap not far from Cobbleston itself.
The cave entrance is located in the top Northeast part of the area.
Upon arriving in the cave, head South past the Save Point and follow the lit torches to the bottom part of the cave to find the plinth. You'll likely have to battle several undead and elemental spirits along the way - but you can always flee to avoid them.
Note that Noelle must be in your party, otherwise you won't be able to make sense of the plinth.
Upon finding proof of King Beowulf's existence, Noelle set out in search of any other legends the monarch may have left behind.