逆行東方 (Gyakkou Touhou, lit. Retrograde/Backward Touhou)
4koma Touhou fanfic series illustrated by Boshi (A-Ieba) with the cooperation of the other circle members. (Tsurukame and Rai)
The webcomic is available at Circle A-ieba K-iu webpage.
Also goes under the title 逆『転』東方 (Gyaku"ten" Touhou, lit. Turnabout/Reversal Touhou) from post #1176886 onwards

See also

• Nico Nico MyList: http://www.nicovideo.jp/mylist/16373151
• Nico Nico MyList(spin-off): http://www.nicovideo.jp/mylist/24250850
• Nicopedia article: http://dic.nicovideo.jp/id/4423991
• Pixiv title search: http://www.pixiv.net/search.php?s_mode=s_tc&word=逆行東方