Memories of Then; The Rise (Merthur)

(Edited Story Description)

Arthur has known about Merlin's magic since they all became knights of the round table, soon becoming a couple. After Uthur died and Arthur became king, he married Guinevere for an heir.
Arthur takes Merlin out to the field for their anniversary after finding a page in Merlin's book the night before. Written on the page says that they can create a child. Merlin uses his magic to do the spell. An accident happens.

Twins.

The fight at Camlann kills many, including Arthur. Merlin leaves the two babes in Queen Gwen's care, her and the others unknowing that she's pregnant with Arthur's legitimate heir. He goes back to Camelot almost 25 years later finally meeting his son and daughter for the first time.

1987. Merlin lives by the lake, his children visiting when the lake splits in half and out walks Arthur. Months of their children story telling and teaching of all that's happened make Arthur cry way too many times. Of course, Merlin sits on the sidelines watching the love of his life catch up to him.