What is the connection between Gisela, the Broken Blade, and Bruna, the Fading Light in the Magic: The Gathering lore?

In Magic: The Gathering lore, Gisela, the Broken Blade and Bruna, the Fading Light are two halves of the same angel named Brisela. They were separated and transformed into individual cards due to a powerful curse. When they are reunited on the battlefield, they combine to form Brisela, Voice of Nightmares, a powerful and fearsome entity.