Complex sentence: However, the diaries that they did find were a fake and, after a battle with Colossus, Shadowcat, and the students, they were forced to return to Sinister with the mission being a failure.
Split into simpler sentences: However, the diaries that they did find were fakes. After a battle with Colossus, Shadowcat, and the students, they were forced to return to Sinister with the mission a failure.