Ye Olde Inn

At the end of each round, where a fight takes place (heroes vs. monsters), and at least one participant (hero or monster) is in a corridor space.

Roll a die (d6) On a roll of 6, a monster appears in the corridor and attacks heroes.

This rule tries to deal with the “gatekeeping”, the tactic where players stay at the door to screen-out monsters.

https://erebus.killerchip.net/homebrew-rules/