Help file included, but specifically this part:
Inspired by the drive for war beneath a bloody banner, the banner's
bearer also gains an extra attack with their weapon.

It does seem this works on NPC's as I certainly notice the damage however gaining an extra attack does not seem to be the case given the large sample of combat rounds with only one attack and no combat rounds that exceed 3 attacks according to the help file 4 should be possible.


'FASHION WAR BANNER' FASHIONWARBANNER
Syntax: fashionwarbanner <PC corpse>

By using this skill, an Orc uses a fallen foe as the source of materials
for a banner used in declaration of war against a given race. Although
the target corpse is not destroyed when this skill is used, some of its
possessions will be used to empower the banner itself. When carried by
a group leader, the banner confers beneficial properties to all Orcs
within the group so long as they are with the banner's bearer. The banner
raises morale, is useful to other Orc skills, and provides increased
savageness to the bearer against the race for which it was fashioned.

Inspired by the drive for war beneath a bloody banner, the banner's bearer
also gains an extra attack with their weapon.