1) just have a game_text be targetted by the trigger_once. Target field goes to name field.
2)If you pick up the items in one map (with good monsters) and then transition to another map (with the bad monsters) you can have this happen. If you really want it all in one map, you can tediously target each and every monster in the map with a multi_manager and teleport them into a room far away, and then spawn in evil monsters when the player walks over an item.