From San'doria, you'll want to exit to West Ronfaure and head to the NW corner and zone to Ghelsba Outpost. From here, there are various ways you can go. The easiest way to explain it is, head to the same cave as where you find the San'doria dragon, but instead of going to the dragon, take the other path and you'll end up in Horlais. Note that in order to get to Horlais you'll need to go through Yughott Grotto, which is filled with Orcs.