Helping Deek, the house elf who stays in my Room of Requirement, is optional. However, until I talk to him I have only one outdoor space for my beasts. As I need to feed and brush them in order to get armor upgrade materials, it’s a small pain to rotate them in and out. The beast room can only hold four species at once. An oceanside space opens up after interacting with Deek.
Also, I’m still barely holding my own with leveling. To give myself a buffer, I pursue his quest to rescue his abused house elf friend. Spoiler: his buddy is dead, surrounded by spiders.
(I’m not enthusiastic about fighting game spiders, but I can do it. Tough mission for those vulnerable to arachnophobia, though.)
Pro-tip: Directly below the cave, glowing green, is the unicorn den. They are the hardest of all the beasts to trap. Open the sack, then stop two or three times and slam the unicorn to the ground again before resuming.
(I didn’t intend to pursue beasts this playthrough, but trapping them is an easy way to level up. The armor upgrades, which rely on a variety of beasts, are pretty useful, too. Without meaning to, I’ve filled every beast space.)
When I return to tell Deek about his friend, the next beast mission opens up. If I trap a pair of thestrals and buy a breeding pen I can make cutie-pies. To keep them happy, the Room opens a third area, a swamp.

The kneazles win the prize for most precious offspring.
And I am now leveled up enough to proceed.
Sebastian has recognized the location depicted in Isidora’s first triptych panel. It overlooks a mine swarming with Ranrok’s Loyalists. We’ll fight our way in and see why this view is important.
We find an isolated room in a mine filled with spiders and goblins. Runic symbols are etched into the rock walls. We wonder why Isidora, with access to her Feldcroft home and the Undercroft, would also choose to use this location.

Another secret mirror wall to the Undercroft is here. We step through and place the canvas we found into the triptych. It’s a location that Sebastian recognizes. He’s ready to go right away, but I want to wait. I’d like to consult with Lodgok first. That I would trust a goblin infuriates Sebastian. He knows that a goblin cursed Anne, telling her that children should be seen and not heard. The tension between us is uncomfortable.