The Ghost of High Heart, who is almost certainly CotF and a beloved friend of Jenny of Oldstones. And Rhaegar was born there the very day of the tragedy.
This is purely speculation, of course, and we're never going to get the answer in the show. We don't know exactly what happened at Summerhall. We don't know exactly when the WW woke up because Westeros ignored the warnings until the last minute, and the Maesters always treated the stories with disdain, because of course the WW were mythological and never really existed in the first place. It could have been many, many years. But Summerhall was a very serious attempt to create new dragons. and House Targaryen nearly wiped themselves out trying.
I was wondering if it was related to the death of the last dragons--that the dragons die and then it's the WW turn, but that doesn't make sense. The WW existed long before the first dragons.
I was just discussing this with a friend a little bit ago and they brought up the fall of the Hapsburgs. Duncan's removal of himself from the line of succession to marry Jenny led to a regime change, a change in the line of descent. Instead of Duncan's line inheriting the throne, it fell to Jaehaerys and his descendants, including Rhaegar and Jon/Aegon. It ultimately led to war and the overthrow of the Targaryens, just like the murder-suicide of Prince Rudolf led to his brother Archduke Karl and HIS son Archduke Ferdinand inheriting...leading directly to WWI.
There can only be 2 people the NK could be after: Bran, because he's the Three Eyed Raven; or Jon, because he is of Rhaegar's line, a direct descendant. Rhaegar thought the PtwP was himself, then changed his mind.
I wish we knew what prophecy Rhaegar got his information from. Wouldn't that be nice? LOL
Edit: You were looking for a birth. What about Rhaegar?