Excerpt taken from Wiki:
Kamen Rider Skull (仮面ライダースカル Kamen Raidā Sukaru ) is a fictional character who appears in the films of the 2009-2010 Kamen Rider Series Kamen Rider W. Skull first appears in the film Kamen Rider × Kamen Rider W & Decade: Movie War 2010 in the portion Kamen Rider W: Begins Night, and then makes appearances in the Movie War 2010 portion, Kamen Rider × Kamen Rider × Kamen Rider The Movie: Cho-Den-O Trilogy in the Episode Yellow film, a music video, in Kamen Rider W Forever: A to Z/The Gaia Memories of Fate, and a starring role in the Kamen Rider Skull: Message for Double portion of Kamen Rider × Kamen Rider OOO & W Featuring Skull: Movie War Core. Kamen Rider Skull's design motifs include the skull and the letter S.
Skull's First Transformation in HD \m/
Credit to JediMaster31 for this upload.