The travel device inside the Area 52 safe room that can take Carl and Donut anywhere they want.
Planning the exit from the Nest Tower, Carl laid out the chain: with the Teleport Ban lifted, The Tower Net was no longer an issue and the escape hatch behind them would drop him and Princess Donut just outside the safe room at Area 52, where they could go in and use the doggie door to go anywhere they wanted.
That freedom of movement matters immediately, since Eris has just told him to find Paca at the Midnight Market and retrieve the Gate of the Feral Gods.
The unrestricted travel device that makes Carl's post-battle objectives reachable.
Also appears in books you've already read — spoiler-safe to their endings: