Terror Unit causes Terror.
Troops who are confronted by monsters or situations that cause terror must test to see whether they overcome their terror. If they fail, they are completely terrified and are reduced to gibbering wrecks. Troops only ever test for terror once in a battle. Once they've overcome their terror they are not affected again.
If a creature causes terror then it automatically causes fear as well, and all the rules described for fear apply. However, you never have to take a Terror and a Fear test from the same enemy or situation - just take a Terror test: if you pass the Terror test then you automatically pass the Fear test, too. As any unit of Troops only ever takes one Terror test in a battle, any subsequent encounters with terrifying monsters or situations will simply count as fear.
A unit must make a Terror test if charged by or wishing to charge an enemy that causes terror.
A unit must make a Terror test at the start of its turn if there is an enemy which causes terror within 6".
Cauldron of Blood The Cauldron and its Guardians are considered to be a single entity and have a Unit Strength of 3. The Cauldron cannot move and its Guardians must remain within 2" of it. The Cauldron itself cannot be harmed. Any shooting hits scored against the unit are randomised between the Guardians and Cauldron in the same manner as war machines (discounting any hits against the Cauldron itself). The energies of Khaine surround the Cauldron, giving the guardians a 4+ Ward save against missile fire (including magic missiles), and Magic Resistance (1). In hand-to-hand combat the guardians are assembled in front of the Cauldron in the same way as a war engine crew, fighting with any enemies in base contact. The Cauldron cannot be attacked. The Guardians cannot be broken in combat. If all of the Guardians are killed, the Cauldron is considered destroyed.