I'm aware of Fatality's success in the CLW era, but unlike other top clans, Fatality slumps for years at a time. This is very uncharacteristic for a "Top 5" clan, and the only reason I rank them lower than #3. Peak Fatality would wipe the floor with any clan, including peak FOE/MM, but unfortunately they're very rarely in this form.
Now add this to the fact Fatality has heavily relied on other clan's closing into them (Tr, Frenzy, CPR, etc) in order to reach the top and when they either left or became inactive Fatality crumbled.