This is an extreme form of Fear and the test is taken in exactly the same way. Only creatures which are mind-numbingly unnatural and horrifying will cause Terror - as indicated in the Bestiary.
Characters failing a Terror test will drop everything and curl up into a gibbering ball until whatever caused the Terror is destroyed or goes away. They may do absolutely nothing, not even defend themselves in combat, and count as prone targets in combat.
Furthermore, Terror is so mind-wrenchingly traumatic that it can cause permanent mental damage, occasionally resulting in personality disorders. Every time characters fail a Terror test, they gain 1 Insanity Point.