This is a debuff that increases damage done by 5% per stack, up to 20% after 4 stacks. Since debuffs dont disappear when you join arenas, you can carry that debuff into arenas.
This debuff is given by a mob called Darkclaw Bat
The debuff lasts 30 seconds, and arena prep lasts for 45 seconds, but you can join the arena just before it starts and you can carry the debuff in fine.
Enjoy your free damage!