The nopon group in the marshes will give you a quest with that item you found in Bionis leg. (the Giant's mirror).
the quest is done in multiple steps:
- Talk to the nopon group in Sarotl marshes.
- Save the nopon in the lizard's men castle. Either come from the main entrance, either from the back door (easier, no fight at all). Too bad, she already talked to them and revealed the secret.
- Go back to Nopons group.
- Go kill a group of 5 lizard's men under the castle, and grab the key.
- Return to castle, there's a secret entrance to the roof behind the king's chair. (Enter from the rear door if you don't want to fight the boss each time)
- Examine the altar,
- Report to Nopon, she says you need a mirror from an old temple on Bionis leg (you already have it. If you don't, go to Bionis's leg, it's located at the west of the lake)
- Return to roof and place the mirror on the altar. You obtain a Giant's hidden relic :
Giant's guts. And now .... run ! You don't have enough levels to fight that
- Return to Nopon, she informs you about the Giant's hidden relics and tells you that the quest will continue later on another place of Bionis.
It's too bad that we can already do that quest but don't have the level to fight the spider boss
It should be the spider which give the mirror, it would be more logic to fulfill the missions after you beat it only.
Edit: I return to Bionis leg at level 37 and could beat that spider (it was hard!).