They have changed it on ZS.. the reward is just chance.. this is my response from support and if I'm reading this correctly if we summon 2 bosses on our own, we're only guaranteed a drop from the first one we summon, the second we'll be in that random chance like everyone that decides to help us. They don't have the common and superior drops like VC and PC. Thanks for contacting us. Everyone who does at least 1,000 healing actions and 1,000 overall actions (this is a combination of attacking and healing) is rewarded with Cash, and XP when the Raid Boss is defeated, however the chance to get a a drop is random. Under special conditions a player may be guaranteed a Superior item drop. For example, a Slayer who summons their very first Raid Boss will get a drop when it is defeated. I'm sorry the communication around this wasn't clearer. That said, myself and the rest of the team here in community and support are petitioning the developers to have common drops along with those superior drops - essentially so the superior drops would still be rare and random, but that every player that contributes minimum actions will at least receive a common item drop along with the Cash and XP reward. That is not something that we'd be able to add mid-event (at this point), but we'll push for it for the next time there is a Raid Boss that visits Zombie Slayer.