The Xbox 360 HD-DVD Player was an add-on released for the 360 in 2006.
It’s purpose was quite simple, and that was to play the new HD-DVD format. HD-DVDs were in a format war with Blu-Ray, and you don’t have to guess who won the battle of supremacy. Now HD-DVD ranks among things like Laserdisc and Betamax.
The player sold for $199 if I remember right, and that price quickly plummeted. Microsoft didn’t even sell 350k units by the end of it’s life cycle. Making it quite a failure of an add-on.