The horn to wake up the stag guardian is infested with fungus.
What to do?
I have it
What to do with it?
Shroomi, at the cave of potions, has an answer.
Find the three nattablooms and put them into the cauldron.
You now have the potent poison to deal with the blocking fungus.
Shroomi said you have to deliver it deep for it to take hold.
Combine the poison with your dagger. Hit the fungus with your poisoned dagger to free the horn.