While pondering on ideas for the Purore region, I came up with a new type of weather - something that I think could be really interesting.
Once we're away from Terasteel- I mean Terastalization, Fairy will probably return to dominance, in addition to the addition of Love-type nerfing the Steels it usually worries about. So to prevent Fairy from getting TOO dominant, here's a weather that not only nerfs it, but also adds a huge boost to the struggling Poison type. Once a Poison type gets some Acid rain up, not only will you struggle to remove Poison types from the field due to their essential free Leftovers, BUT their Sludge Bombs and Sludge Waves will hit even harder.
And an update to the Poison Typergy ability, Ultra Toxic: not only do Normal-type moves become Poison Type, and not only do all Poison moves do double damage to poisoned foes, and NOT ONLY can all Poison Type moves damage Steel types...but the ability also calls forth Acid Rain, which can't be removed until another Typergy ability brings in new weather.
...I think I just made Poison types completely broken.