A great harvest.
Duncan now has plenty of Legendary Level to spare, so it shouldn't be difficult to engrave any ability he wants. It's fair to say that the bulk of the Legendary Levels from the Battle of Sharon went to him, Aetius, and the king and son of the Western Goths. If Duncan had not actively altered the course of history, most of the Legendary Levels would have belonged to the king and son of the Western Goths and Aetius. Now, the Legendary Level gained by Aetius should be less, with some directly transferred to Duncan, likely lowering his historical evaluation significantly.
Duncan managed to snatch some achievements from both sides, netting a big catch in one go.
There's no need to store up too much Legendary Level.