Okay, I'll admit I'm just posting this because I love the old school NES style box! This is the 'Tools of Hero Kind' expansion.
From the Publisher:
"The adventurers strike back! In the original Boss Monster, Heroes are puny resources to be gathered and slain. But in Tools of Hero-Kind, they put up more of a fight! This mini-expansion introduces new item cards that make Heroes more formidable. Defeat these heavily armed Heroes, claim their Items, and gain new offensive abilities!"
Looks pretty cool. I wish I knew more about the original game. Might check this out more. I'll let you know what's up.