Talk.

From TYPE-MOON Wiki

"Talk." is a text-based story, the first after the epilogue in the Tsukihime game to hint about the possibility of a sequel to the main storyline of Tsukihime. It features the characters Ciel (after losing her immortality after Roa's death), Merem Solomon, Tohno Shiki in his appearance known as "Satsujinki", one of the Twenty Seven Dead Apostle Ancestors Einnashe, and Forte, a magus from the Association.

When it was published, Nasu mentioned that he wasn't certain about making it canon yet, meaning that this side-story could be dropped and forgotten. However, the Prelude short stories published in the Character Material seem to be heading in the same direction.