The attack of siege weapons like Trebuchets or Ballistae is called a Siege Attack. It deals a large amount of damage to all creatures in the impact area. Players can first be hit by Siege Attacks in the Fort Ranik mission (as friendly fire), other than that notable places to watch out for Siege Attacks are the Frost Gate mission and the Ice Caves of Sorrow mission. It is worth noting that there are essentially two different versions of this skill: one causes severe damage, to the order of 150 damage against a 60-armor target, the other, caused by Trebuchets, causes lethal damage, capable of killing any character at any level with nearly any amount of armor without health boosts from Vital Boon or similar skills.
Also see Wurm Siege.