Udam Homelands
After Sayla gives you the antidote recipe, You can now craft it to get past the Rot Fumes in the entrance to the Udam land. Head to the Big Darwa Fort and follow the path that leads to the cave with the green smoke. Take an antidote by going into the food menu (D-pad Left) and selecting it. Eating it will give you a buff which removes and negates poison effects. While the buff is active, head into the cave, deeper into Udam territory.
The canyon, and of course, the homeland, is littered with lots of Udam units so take care in getting spotted or you could be overwhelmed (If you have all the Skills then you can probably take all the enemies you can).
There are also a lot of heavy units around so the best way to deal with those is a fully upgraded spear which you throw at them. They usually go down easily with those. Just remember to retrieve the spears you throw.
After exiting the canyon, your next task is to capture the bonfire in the middle of the homeland. Head over there and do what you normally do with Bonfires. Use your scout and clear all the enemies.
When the bonfire has been lit and captured,
your next task is to find Ull's cave, It will be marked on the map and it is to the north.