If this event is similarly triggered to the others, it appears to be linked to a single dungeon randomly chosen on each map (or the roll to trigger happens only the first time someone meeting the conditions enters each dungeon, roughly the same result from the player's perspective). This one only ever triggers at night. Singleplayer appears to increase the odds of random event triggers, although this could be confirmation bias. Having more than 0 rot might also increase the odds. My advice is to play a high-fight hero (to better kill banes, which have a higher chance of spawning on explore at night), getting at least one rot, and trying to explore every single dungeon on the map at night for a few games. Maybe Fang with the Celestite ring (or Thane with the rot amulet) for extra ease of movement? Best of luck to you!