There were two babes in the Tower of Joy.
Two babes tied so closely to their parents, Ned had to make the decision.
One with hair like raven feathers, eyes as grey as steel, and a face of a true Stark. The other, with hair like moonlight, eyes like an indigo pool, and every delicate feature that the Targaryens held above all else.
Aegon and Visenya.
The prince(ss) that was promised.
Rhaella made a choice that day Rhaegar's heir arrived on Dragonstone; to forever hide the parentage of Visenya Targaryen and to cut her completely from the prophecy her son was so dedicated to. Visenya Targaryen, Daughter of Lyanna and Rhaegar Targaryen was no more; in her place grew Rhaenar Waters, bastard daughter of The Mad King and soon to be heir apparent.
Incest warning, major character deaths, gore... typical Game of Thrones warnings
Runs on a similar line to Daenerys but with changes and different pairings and decisions.