The Iron of Yin and Yang: Buront's Entrance into Gensoukyou (Japanese: 東方陰陽鉄 ~ブロントさんが幻想郷入り~) is a series of Touhou videos made using RPG Maker and uploaded to NicoNico Douga, featuring crossover elements from the MMORPG Final Fantasy XI. These videos became surprisingly popular, and gained enough of a following to spawn a significant amount of fanart based on its themes.
Short summary: After saving his linkshell from a King Behemoth, an Elvaan knight named Buront is catapulted into Gensokyo, landing directly in Reimu's donation box. From there, Buront becomes part of several new adventures, encounters old enemies, and generally annoys people with his knightly boasting.
This tag should apply wherever scenes or characters from the video series are depicted, or when the artist's comments and/or tags (on pixiv, for example) indicate the relation.