Lord El-Melloi II Sei no Jikenbo: Rail Zeppelin Grace Note - Tokubetsu-hen

 A letter is delivered to Lord El-Melloi II informing him of a Clock Tower reunion. One morning, as the reunion's date draws near, Lord El-Melloi II takes a look in the mirror and reels in shock. He is suddenly a full decade younger! Lord El-Melloi II has become Waver Velvet, his former self! Believing himself to be in the grip of a powerful, magically induced delusion, Lord El-Melloi II immediately begins investigating how this could have happened. On the day of the reunion, he makes his way to the gathering of his contemporaries, planning to confront whoever transformed his body and hoping to reveal their intentions. However, when Touko Aozaki arrives, things take a sudden turn...

(Source: Crunchyroll)

Note: The special edition was broadcasted in the Fate Project New Year's Eve TV Special 2021.