
The concept of “if you're infected by the Flood it's basically an eternal Hell” has been, if not outright said, implied since Halo 2.
In the famous scene where the Gravemind basically reanimates Regret to prove a point, the only way that makes sense is if there’s some mechanism by which the Flood can “retrieve” Regret’s consciousness even after the prophet’s physical body is dead. Because keep in mind:
- The Chief had beat Regret to death at least several hours before the prophet was infected.
- The building was also glassed.
Also take note of how Regret starts screaming when the tentacle gets lowered down, implying that the Regret the Gravemind reanimated isn't just the Gravemind using the prophet’s voice, it's the actual Regret and he’s fully aware of what's being done to him.