Only heroes have to fight fair.
There was a time when heroes roamed between the Four Great Cities. They fought for kingdoms and princesses and they lived and died by their valour. Or so the stories say. The way the minstrels sing it, everyone had a swashbuckling good time.
Nowadays, buckles are rarely swashed. All the great beasts have been slain, all the great treasures unearthed. Princesses are few and far between — the ones worth fighting over, anyway — and the kingdoms have been divvied up between the sons of sons of sons of the heroes of old. No valour required.
Frankly, the hero business is in a sorry state. Luckily for Grin the Cheat, thieving and murder are thriving as usual.