Halfling Scales of War Campaign
The party found a strange brass key on a Dark Creeper while trying to close The Nexus. Later the party was attacked by a group of Lost Ones looking for the key. While exploring underneath The Happy Beggar the party found a second identical key on a Shadar-kai witch. A short time later one of the keys was stolen by Modra, who used it to open a portal to the Shadowfell.
A few days later the party killed Modra and recovered the 2nd brass key.