I’m not sure I handled that wyvern fight in the best way. An option I was considering was just having the wyverns fly away into the darkness, since they had no real chance of defeating the party. I didn’t do that partly because I find inconclusive encounters to be deeply unsatisfying, but also because it didn’t fit the image of wyverns as the violent, territorial creatures described in the bestiary. I settled on them doing the only thing they could do to inflict meaningful harm: try to carry off the smaller party members. It required them to make really good combat maneuver rolls, and unfortunately for them, they didn’t. Maybe it was good up until there was only one wyvern left, and then I should have had the last one run off? Maybe.
Well, at least the rot grubs were scary.