Mirwen was pleased with the texts you brought her, but was also unable to immediately translate them.
'I fear you must venture back into Moria and search the deepest reaches of the Water-works. Scour the waterways that give the area its name.
'An ancient toad, named Greatroar, was known to be attracted to items of great knowledge. If you should find her, slay her. The book is as likely to be in her belly as anywhere else in Moria, if the creature still exists!
'Once you have acquired the book, take it back to Winiel. She will be able to translate these writings with the aid of that book.'
Greatroar is somewhere in the Water-works of Moria, far to the south of Rivendell.
Mirwen told you to find the toad Greatroar and defeat her, for the toad long ago swallowed a book of dwarf-lore.