The Fire & Blood chapter "Jaehaerys and Alysanne, Their Triumphs and Tragedies" details the striking death of Aerea Targaryen in 56 AC, though there is a great deal of mystery surrounding this event. Aerea returns from an unknown location, "Balerion had wounds as well", and Grand Maester Benifer who was said to record everything he witnessed is silent on this event. All that we have is an account from Septon Barth.
The things... Mother have mercy, I do not know how to speak of them... they were... worms with faces... snakes with hands... twisting, slimy, unspeakable things that seemed to writhe and pulse and squirm as they came bursting from her flesh. Some were no bigger than my little finger, but one at least was as long as my arm... oh, Warrior protect me, the sounds they made...
-Septon Barth, Fire & Blood
What was this creature that killed Aerea Targaryen?