No Retreat!
(Rule)Description: It’s not uncommon for units to be immune to Morale checks for losing an assault, or to automatically pass them for some reason (they may have the ‘fearless’ special rule, be subject to a vow or some other special rule). When such units lose a close combat, they are in danger of being dragged down by the victorious enemy despite their determination to hang on.
These units do not take Morale checks and will never fall back. Instead, these units suffer a number of wounds equal to the number their side has lost the combat by (allocated as normal). All types of saving throws, except for cover saves, can be taken against these wounds.
If none of the enemies involved in the combat against a fearless unit can actually hurt it, the unit does not suffer any wounds if its side is defeated in combat, and simply continues to fight.