Complex sentence: Serenity is an Austrian symphonic power metal band formed in 2001, and later in 2004 became a more stable group with a consistent line up, using more progressive and power influences than earlier.
Split into simpler sentences: Serenity is an Austrian symphonic power metal band, which was originally formed back in 2001. However, it became a more stable group with a consistent line-up in 2004, when it also began using more progressive and power influences than earlier.