A Darker Shade of Magic (Shades of Magic, #1)

by V.E. Schwab

“It’s called a soul seal. Spells can be broken. A soul seal cannot. It’s a piece of permanent magic. But this,” he added, grazing the mark, “this is…” “Forbidden?” ventured Lila. “Impossible,” said Kell. “This kind of magic, it doesn’t exist.”