Right.
Then I either am being very stupid or have some sort of problem, because it did not let me do that. I have plenty of legendary items. I choose to keep the items (which made Llengrath miffed - I felt rather guilty about that actually, the first time) and the godseeds remained quest items in my inverntory and there was no option to craft to mythic in the enchanting menu.
So, let me, for the sake of arguement, walk through this (and then, if I'm not doing something stupid, I have something to take to the bug report thread.)
Fortunately, being a clever sort of Lich, I have the appropriate save.
(Of course, I gave up, gave the seeds to Llengrath, seeing as no-one had any answers and I carried on from this for several hours into Beast of Winter before making the mistake of buying Stardew Valley as it was on sale and thinking it would be a nice little time killer and not that it was so damned addictive and thus halting my playthrough...!)
So, pre-quest completion:
Godseed in quest items.
Talk to Llengrath:
Picking the quickest "no tou can't have it" option for expediency (when I did it proper-like, I went through all the dialogue).
Upset poor Llengrath...
No option to enchant...
... Still in the quest items.
The only thing I can think of that I might be doing wrong is if you can somehow give her the items and take them back off her, but from what I recall, if you give them to her, she just goes "ace! Ta very much!" and quest completed.
On the plus-ish side, my terribly Lawful Good-ish watcher doesn't feel guilty about upsetting her by not giving them to her. For some reason, no idea why, her reaction felt like kicking a puppy and I don't even know why! (I mean, most of the time I have no particular puppy-kicking objections, not being A Pet person...!)