Complex sentence: But after their second leader was sealed away 400 years ago by the Kakurangers' ancestors, This forced the powerless Youkai to go into hiding and change their forms to keep up with the times.
Split into simpler sentences: But after their second leader was sealed away 400 years ago by the Kakurangers' ancestors. This forces the powerless Youkai to go into hiding changed their forms to keep up with the times.