Chaplain Cassius

Chaplain Cassius and any Ultramarines squad led by him benefit from the Preferred Enemy (Tyranids) special rule.

Any Ultramarine squad within 6" of Cassius may re-roll their Morale checks, accepting the second result as final. Note that this ability can be used even if a Morale check is passed on the first roll, so you can use it to try to make sure Ultramarines fall back in disadvantageous or dangerous situations.

Chaplains embody the honour of their Chapter. They, and all members of any Space Marine squad they have joined, lead, or attached to are Fearless.

On a player turn in which he charges into close combat, a Chaplain, and all members of any Space Marine squad he has joined, leads, or is attached to may re-roll failed rolls to hit.

Space Marines automatically pass Morale tests to regroup, and can take such tests even if the squad has been reduced to less than 50% by casualties, though all other criteria apply. If Space Marines are caught by a sweeping advance, they are not destroyed and will instead continue to fight normally. If this happens, then the unit is subject to the No Retreat! rule in this round of close combat and may therefore lose additional casualties if outnumbered. Usually troops that regroup may not move normally and always count as moving whether they do or not, but these restrictions do not apply to models with this special rule.
Units that include Servitors and Familiars are still subject to this rule as long as the unit contains at least one Space Marine.
Space Marines are still subject to Last Man Standing tests. They will always regroup after each Fall Back move to 'reassess' the situation however.