I think that there are at least 2 different ways that this battle can occur, depending on how you interact with the doppelgangers at the start of the area.
1. You don't recognize the doppelgangers for what they are. You get ambushed outside the cave by 1 white dragon and the wyrms. The 2nd white dragon remains inside the cave.
2. You DO recognize that there's something wrong with the "doppelgangers". You kill the doppelgangers. You go inside the cave and run into both white dragons and the wyrms.
Either way is very nasty. In the 1st scenario, you don't get any preparation time. In the 2nd scenario, if you want, you have plenty of time to prepare. Just expect to have a nasty battle on your hands.
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