Source: Guildmaster's Guide to Ravnica

Wondrous item, uncommon (requires attunement)

This headpiece, crafted from dark metal, is worn curved around the ear. If you know a creature wearing another spies' murmur and that creature is within 1 mile of you, you can communicate telepathically with each other. As a bonus action, you can allow that creature to hear everything you hear for 1 hour. You can end this effect as a bonus action, and it ends if you're incapacitated.