This illustration was done by:

Illustrated by Mizuki Kato — Works as a 2nd key animator for "Monogatari Series: Off & Monster Season" from what I could find.

Supervised by Akio Watanabe — The character designer for the Monogatari series.

Finished by Shaft

You can find this illustration in the September 2024 issue of Newtype Magazine.
On the front cover of Newtype Magazine September 2024, it mentions this illustration is included as a B3 poster. Available now (released August 8th, 2024) in Japan.

from: https://www.reddit.com/r/araragi/comments/1epk8dj/monogatari_series_off_monster_season_illustration/