“There is power emanating from it.”
“The horns make it really easy to turn.”
“Little key is for make door.”
“I think I know where it goes...”
“FOR ACTIVATING, NOT UNLOCKING”
“Got it from that big bony hoser.”
“Heimdallr's key tö the Bifröst.”
“This key is REALLY old.”
The Ancient Key is an item exclusive to Don't Starve Together, introduced in A New Reign. Its sole purpose is to activate the Ancient Gateway (and subsequently the Atrium Biome) by being inserted in its socket.
The Ancient Key can be obtained from a Large Ornate Chest after killing the Ancient Guardian.
Wickerbottom with the Ancient Key as seen in the Forge cinematic.