Idia explains that his unique magic Gate to Underworld (hidden meaning: the Opened Door of the Underworld) is hereditary: “People born into the Shroud family possess a blood curse that burns blot. And they have a unique magic that opens and closes the gate to the Underworld.”
We have seen Idia use his unique magic three times: once to open the gate to the Underworld in Book 6, once to close it again, and once to open it in his dream in order to escape the abyss in Book 7.
Twisted Wonderland Reference Note 