Alright, things that are true and/or happened alot in the chadztext:
-It's not kills related, as chadz stated.
-It's not damage related, as chadz stated.
-It seems it is only for the losing team.
-If you get it, you will not lose your multiplier, or gain a multiplier as okiN said.
-All the people who got it were the last guy alive OR one of the last guys alive.
Now, let's look at the text itself.
"Thy valour on the field of battle hath been rewarded"
What i think, is that you are courageous even when you are outnumbered. You try to even the odds again when you are losing, and your team is almost dead. Succeeding in that will reward your daring action with a multiplier/lesser multiplier penalty.
This makes people still to fight on, instead of to go kiting as an archer.
valour - the qualities of a hero or heroine; exceptional or heroic courage when facing danger (especially in battle); "he showed great heroism in battle"; "he received a medal for valor"
Heroïsm.