The portal was condensed by expending all the corruption that was in the Aberrant World.
Rhizen's Tower of Purification—Pearl Buck—had the property of shielding whatever world it was planted upon from the senses of Apocalypse, preventing it from interfering with this world.
Moreover, it also prevented the corruption in the world from making a connection with the System World, the place where Systems were birthed.
As all System Skills regarding that were rendered useless, the corruption had only one choice, and that was to consume all the corruption in the Aberrant World to brute force a way through the world.
As the Tower of Purification had multiple abilities, one of which was to act as a beacon for trans-universe travel, by pouring all the corruption plaguing the world through it, Yulna penetrated through the fabric of space of this universe, creating a portal into the adjacent universe.
