An “event horizon” is the point in which the gravitational pull of a black hole is strong enough that light cannot escape. So when she says she is hiding secrets under the events horizon means she is probably near a black hole.
English
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She did bring her forces into a black hole ( I think it’s because she tried to find a way to break away from the worm’s hunger) for a long time, so it’s not out of the picture
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I’m not ruling out the astronomy possibility, but other clues in other passages as well as the foreshadowing being done by bungee themselves points at my statements above. The event horizon is a play at words to signify point of no return essentially to an observer. This is meant to make you question who is observer and observed as well as what point has been reached with regard to savathun and oryx. My guess is more of a parallel with the dreadnaught and the planet titan.