Solar eclipses. Just got my first one today, and was hoping to get at least one broken hero sword from a swamp thing of Frankenstein, both drop at a rate of 1/250. But no, RNG decided it didn't want to give me a broken hero sword, just other rare items. Like three swamp thing banners, which drop at a rate of 1/200. The chance of t is TINY! Luckily, though, it redeemed itself, and I got a 1/250 drop from a reaper, which are much rarer than either of the other two, and got a Death Sickle.