Hydreigon can only be found in Sword when not part of a promoted event. With that said, here are the dens it can be found in:
Den 36 (Bridge Field)
Den 63 (Bridge Field)
Den 36 (Dusty Bowl)
Den 88 (Dusty Bowl)
Den 36 (Giant's Mirror)
Den 63 (Stony Wilderness)
Den 63 (West Lake Axewell)
Source
Here's a handy little picture you can use to find those dens:
There's also currently an event going on that makes Hydreigon more common in dens.
Hope I helped!