There are many instances of terroristic actions taken by people throughout history. In some ways, though, enumerating them brings up the problem of how to define terrorism.
One way to define terrorism is as acts of violence that are perpetrated against civilians who are not involved in government in order to further a specific social or political goal. In other words, terrorism attacks people who are not the decision-makers in a society and does so in an effort to force the decision-makers to make a certain choice. The problem with this definition is that many acts then become terrorist acts. The dropping of atomic bombs on Japan in WWII become terrorist acts, for example.
We can also change the definition of terrorism to say that it cannot be carried out by overt agents of a state. In other words, the atomic bombs would not count because they were dropped by the military and were therefore military actions, not terrorist ones. From this point of view, terrorism would be confined to events like the Palestinian killings of Israeli athletes at the Olympic Games in Munich in 1972.
Thus, it is hard to list instances of terrorism because it is hard to define exactly what they are. Most people would say that the Palestinian attacks qualify. Most people would say that the 9/11 attacks in the US and the bombings of subways in Madrid and London would qualify. Most people would say that the Aum Shinrikyo gas attack on the subway in Tokyo would count. Other instances are less clear. Were the pogroms carried out against Jews instances of terrorism? Was the Spanish Inquisition an instance of terrorism? These are hard questions to answer. Therefore, it is hard to list terrorist actions because people might not agree with your definitions of terrorism.
the september 11 bombings would have to be one of the most drastic attacks on civilisation.