While technically a side quest, saying that it’s optional may be a stretch. After you leave Oleg’s Trading Post for the first time, your protagonist may - after resting a few times in the Stolen Lands - start having nightmares. Your actions in this Illustrated Book Episode do not matter, it merely serves as a summon to Shrike River - more specifically to Nettle’s Crossing. You can ignore this at your peril, but so long as you do, your protagonist will not rest peacefully, meaning you’ll keep the persistent effects of exhaustion and you won’t recover expended spells.
Make your way to Nettle’s Crossing by following the recommended first expedition in the walkthrough, or a bit more directly by following these directions (from Oleg’s Trading Post):
From where you arrive, follow the road southeast until you find a trail running to the northeast alongside a burned-down house. Follow the trail past the house, continuing southeast to approach the shore, where a drowned corpse will pull itself out of the water to confront you. Ask the corpse who it was and how it died to recall a vision similar to the dreams you may have had. In any case, the corpse was slain by the Stag Lord, and wants vengeance… vengeance that’s beyond its power to acquire. Agree to avenge the corpse - it’s the only way to get the dreams to stop - and it’ll offer up the spear it still carries should you come back successful. Attacking will only temporarily buy you a reprieve from the visions, so even evil characters are subject to the corpse’s irrepressible wishes.
From this point on you’ll be free from the corpse’s intrusive dreams, and this side quest will run parallel to this chapter’s main quests. You’ll have to resolve your business with Tartuccio and the Stag Lord, after which (presumably on your way back to Oleg’s Trading Post to tell him of your victory) you’ll need to stop by Nettle’s Crossing.
Do so and return to the river, where Davic Nettle will pull himself out of the water, mumble to himself, and give you your rightful reward - a Longspear +1.