Complex sentence: On December 12,1915, Yuan agreed to become the next emperor and proclaimed himself Emperor of the Chinese Empire (中華帝國大皇帝) under the era name of Hongxian (洪憲; i.e. "Constitutional Abundance") beginning January 1,1916.
Split into simpler sentences: On December 12,1915, Yuan "agreed" to become the next emperor. Yuan proclaimed his reign as Emperor of the Chinese Empire (中華帝國大皇帝) under the era name of Hongxian (洪憲; i.e. "Constitutional Abundance") beginning on January 1,1916.