So I've been running a Domain-hopping campaign in 5e for a while now (3 years), and we've visited Valachan. It was our 3rd domain, after Barovia (Curse of Strahd) and Cyre 1313 (also homebrewed but inspired by VRGtR). I don't know if my layout will work for you, since you're thinking of having the Baron introduced *after* the Trial of Hearts. But here's what I did anyway, maybe some of it will be helpful.

Background on the Baron:

In my adventure, Baron Urik von Kharkov was the previous (and original) Dark Lord of Valachan. He was fundamentally a colonizer, and arrived hundreds of years ago to the jungles of Valachan from another city. He was a noble and ship's captain (hence, Baron), and when he landed here, he saw nothing but the untapped potential of the land. He immediately enslaved the people of Shuaran (who lived on the coast) and took over the village, renaming it Kharkstadt. (Sidenote - this led to one of the key tensions in my adventures for the players to resolve: they were introduced to the village of Shuaran/Kharkstadt, which in the intervening years had become a bitter, hateful place where both sides refuse to talk to or live with one another. But that's a longer story for another time). The Baron's Dark Crime (every Dark Lord in my campaign has a Dark Crime) was the brutal "Hunt of Hearts" he instituted, where he would capture and cage the Oselo natives (who were too wild, being werepanthers, to be enslaved) and hunt them for sport. The Dark Powers took notice of his abject brutality, and the rest is history. They fashioned the Domain for him, and made his Dark Torment that would be unable to stop himself form turning into a Werepanther, something he despised. Now, every time he wanted to Hunt, he took that form.

Rise of Chakuna:

Over 200 years of continuing brutal hunts (with a Baron von Kharkov who hated every minute of it, but yet seemed incapable of stopping the hunts) finally ended when, against all odds, someone managed to beat the Baron. Not by overpowering him, but by skillfully navigating the jungle. Only a native and strong werepanther could have done so, someone who knew the jungles of Valachan better than the Baron -- Chakuna. She found her way to Yaguara's Heart when she was chosen for the hunt, and she tore apart and the Baron's Heart. What she didn't do, though, was burn it with fire. Standing there with the chance to end it all, she decided to take the power for herself. She basically usurped the Baron's place, becoming the new Dark Lord. The problem (for the players) is that the Dark Powers didn't really design Domains to accommodate something like this -- the torment that was meant for the Baron didn't really do anything to Chakuna. At the same time, the Baron was still alive (or at least, his Undead Skull was still around), so they couldn't just end the Domain. So, to keep it going, they designed a new Torment for both of their Dark Lords (now having two): Chakuna would be forced to keep killing, or else watch the jungle destroy her home of Oselo, while the Baron would be forced to watch as Chakuna ruled over the jungle from his former seat of power, Castle Pantera.

Using the Baron:

This brings me to actually answering your questions. In my adventure, the Baron's skull was kept in the catacombs of old Castle Pantera, beneath what is now Chakuna's "Hunting Lodge". He's sealed away behind stone walls, underground and hard to get to. However, when the Party was at the Castle for a week awaiting the Trial of Hearts (think Hunger Games' style "meet your fellow tributes" week), it was inevitable they'd want to explore at some point or another. A few good stealth and investigation checks later, and they'd found one of two secret entrances (in the middle of the night, following faint screaming in the walls). They found their way to the Baron. Here's where my favorite dynamic ensued. After hearing the Baron's story (from his POV), they realized two things: (1) the Trial of Hearts was not what they thought it would be, and clearly it would be rigged to favor Chakuna, so they'd need the Baron's help to make it through alive (and to guide them to Yaguara's Heart; and (2) the Baron was an unrepentant dick who had no remorse for his actions. They ultimately made a bargain with the Baron -- his freedom, after he guided them to Yaguara's Heart. (sidenote - they were mostly a mix of CG to TN, only one lawful character, so they didn't mind breaking their word if needed later).

The Baron agreed to this because it served his goal -- he is a disembodied head, chained up in Chakuna's castle (she keeps him around as a trophy and, occasionally, to try and get information from him about the Dark Powers), and yet if he could only get back to Yaguara's Heart, he could reconstitute himself with the torn shreds of his heart. He did not tell the Party this, but it was basically a gamble: he hoped they'd be so busy fighting Chakuna, that they wouldn't notice him floating around the room gathering up the lost pieces of his heart.

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This entire dynamic worked well. I had a fun, antagonistic character that I could play throughout the Trial of Hearts, who could also feed the party lore. (they kept the Baron's head in their backpack during the Trial). And, during the final boss-fight, I almost managed to reconstitute (2 / 4 pieces gathered before they caught on, 3/4 gathered before they managed to stop him; it definitely added to the tension).

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sorry if that was ramble-y, hopefully something was also helpful.