大毘盧遮那經供養次第法疏 1.42

1.42

「次為降伏魔」已下，第十五無堪忍大護明門，亦有誦真言。此中降伏魔者有四：煩惱魔：陰魔：天魔：死魔。無能堪忍者，魔鬼等者見此持誦真言人，開眼欲見不能見，故言無堪忍明也。

From "Next, in order to subdue māras" onwards is the fifteenth, the Unbearable Great Protection Vidyā gateway; it also has a recited mantra. Here, "subduing māras" refers to four: affliction-māra, skandha-māra, deva-māra, and death-māra. "Unbearable" (asaha) means that when māras, ghosts, and the like see this person who recites the mantra, even if they open their eyes wishing to see him, they are unable to see him; hence it is called the Unbearable Vidyā.