It's called Obsidian and it will premiere on November 19 on HBO Max. Michaela Dietz will provide the voice of Glassboy.

Set to premiere on May 20, 2021 on HBO Max, Together Again is the upcoming 3rd 40-minute special of Adventure Time: Distant Lands and it will primarily focus on Finn and Jake.

I'm assuming this is going to be their final adventure and sadly, we're going to witness the death of Jake, which would explain why he didn't appear at the end of Obsidian (only as a tattoo on Finn's chest).