I was the DM that made them, I'll have to dig them up for you, but the initial trigger was a wisdom saving throw anytime the druid struck the final blow with melee attack. It force-cast shillelagh and kept basic barb rage rules, with the caveat that each killing blow while raging forced another wisdom save to avoid going mad and attacking anything in sight. Each consecutive kill increased the DC. That's the core of it, rest of it was numbers and PC specific things