How to make underworld?

First you need to build an underworld area in the map, like building a new map. Design it to your own perspectives. Then add a trigger_add_tf_player_condition with condition 108 to where players have the underworld effect. Normally in Viaduct players are teleported to the underworld with an entity thet teleports to a targeted location, but since you are teleporting at the end of the round I think you can use a trigger_mulitple in your regular world that fires when the round timer ends, something similar to Ghost Fort and the Dance Effect on the wheel.