Helaena is sensitive to things others aren’t. So when she’s forced to choose between the lives of her two sons, she feels it. You could say she’s felt it more than once since she might have seen it in her dreams.
Following Helaena’s refusal to yield to Rhaenyra’s claim to the throne at the behest of her mother Alicent, Daemon hatches a devilish plan. He sends two assassins, brutes who’re only named Blood and Cheese, to kill one of Helaena’s sons.
Source: https://www.distractify.com/p/what-happens-to-helaena-targaryen