The Plight of the House-Elf side quest in Hogwarts Legacy becomes available during the Stop Ranrok and Rookwood main objective. Travel to the Room of Requirement to find Deek. Speak with the house-elf to get a new addition to the room. Deek will then ask you to check on his friend Tobbs in a cave near Hogwarts.
Exit the school and travel north to the Spinners Cavern slightly northwest of Hogsmeade and west of Upper Hogsfield. Deal with the spiders guarding the cave entrance then head inside.
Near the entrance, you’ll find a wooden table with a lantern on it. There will be a note here that you can collect. Head deeper into the cave. Eventually you’ll reach a room where a Thornback Matriarch will show up. After you defeat it, there will be another note on a desk next to a lantern you can collect. Continue heading even deeper into the cave and you’ll come across the boss spider, Acromantula and its horde of minions. Defeat them all then continue on to find Tobbs. Return to Deek to give him on update on his friend to complete the quest.