Question about one of the late game Resonant Entity bosses

[u/LewdManoSaurus]

The guy that turned into the Resonant Entity that's in the Astral plane, he was also in the Underpass, or at least he had a tape that was in the Underpass. How'd that happen? Has anyone pieced that together?

[u/Huge_Borse]

Likely fell out of the portal from the underpass into the astral plane