Chime of Warning
This chime was created by the Third Jade Emperor (on the World of the Jade Emperor) in approximately the 12th century. It stands waist high and is made out of jade (as are all magic items from that alternate reality).
The chime cannot be rung and, if struck, will make only a dull thwacking noise. If any creature attempts to pass into the reality that holds the chime of warning (within several million miles of the chime’s location), then the chime lets out a clear ‘dong’ sound. This sound is audible to everyone within one hundred miles of the chime’s location (not because it is so loud, merely because the magic allows all of those within range to hear the sound).