This is how Umarak finds out about Alicorn magic goes in The Magic Hunt.
[Umarak watches various videos on Alicorn magic in his lair. Two have Celestia and Luna raising the sun and moon and the other two have Twilight and Cadance spreading friendship and love]
Umarak the Hunter: So this is how they keep peace and harmony in Equestria.
Umarak the Hunter: With their precious Alicorn magic inside me, I will become a Destroyer. [evil laughter]
[Umarak goes out to get the Alicorn Magic. In her room, Celestia jolts awake]
Princess Celestia: [breathing heavily]
[Princess Luna comes rushing in]
Princess Luna: Sister, are you alright?!
Princess Celestia: I've just had the most terrible dream.
Princess Luna: Why do you think I'm here? You know as well as I that this was not a dream, but a vision.
Princess Celestia: Then we haven't much time. The stronger he becomes, the more we Alicorns are all in danger.
[A few hours later, a book is brought out and flips pages until it stops at a page with Umarak on it]
Princess Celestia: (narrating) Umarak the Hunter and his followers came here from a distant land, intent on stealing Alicorn magic. But most of Umarak's followers soon came to appreciate the ways of Equestria, even befriending a young unicorn wizard.
Princess Luna: (narrating) Most of Umarak's followers urged their leader to abandon their plans. When Umarak refused, his former followers alerted us to Umarak's intentions.
Princess Celestia: (narrating) Most of Umarak's followers returned to their own land, and Umarak was sent to Tartarus for his crimes. But it appears he has found a way to escape.