Doomsday Manual

jHexen: Cannot progress shortly after going through Axe Key Door;

There is a script typo in the original HeXen regarding the Chaos Serpents behind the Axe Door in the Gibbet.

The script checks approximately every three seconds for when exactly three of them are still alive and then lowers the walls of the room unleashing the next wave of monsters. If there are more or less than three alive, the script will not activate.

Needless to say, if the player reduces the number of serpents to less than three without waiting for the walls to lower first, they will become stuck and unable to progress without cheating (the noclip cheat).

It is believed that the script was probably intended to activate when there were less than three left.