General believe is that the main aim of ISIS is to have a territory in order to form their own state where they can live their life according to Islamic teachings. That can be true but I believe that ISIS wants to separate fear and hostility in world. They want to get fame and recognition as a brutal and most scary terrorist group in the world. For example, ISIS took responsibility of several bomb attacks in Pakistan, including a kitchen cylinder blast which was misreported as bomb attack in news. Later investigation found that it was cooking cylinder blast in kitchen. This shows that ISIS deliberately took responsibility of bomb attacks and other terrorist act around the world to terrified people.
What can be the real reason behind separating terror is open for discussion.