The Monster at the End of This Story is a Sesame Street special which premiered on HBO Max on October 29, 2020. The special was an animated adaptation of The Monster at the End of This Book and features Grover, Elmo, Cookie Monster, Rosita, Abby Cadabby, and Oscar the Grouch.
An animated short based on the special was released on September 22 starring Grover and Elmo. (YouTube)
The special was originally to be promoted around a Grover-centric campaign called "The Year of the Monster." The campaign has since changed to focus on all the monsters of Sesame Street, now called "Furry Monsters."