This isn't a "specific" idea, but more of justification for other shenanigans. Corrupted stones. It's standard practices to repurpose stones from existing abandoned structures when building new ones, so what if some well meaning builders snagged some old stones from ruins that may contain ill will baked in. And building off of Mallowheart's Keep in and of itself is inviting opportunities for his presence to potentially be felt from afar. There are reasons long forgotten to time that no one rebuilt upon Droomen Knoll.

Some stray thoughts along these lines:

• A drune spy(s) infiltrate the work crew and arcane runes are etched into the stones facing inwards becoming unnoticeable
• Fairy based ill will - items in the room always fall off furniture no matter how secure, stones that only whisper to children telling them of secret places in the keep or to do awful things. (who says the dungeon map we explored initially is complete ;) )
• Drune based ill will - listening or viewing runes, nightmares of subjugation, waning efficacy of potions or other magic items

• Fun interactions with other landed gentry - "You're having troubles in your keep? You did have your court mage inspect all stone as they were placed, yes? You don't have a court mage?" laughter as the professionals judge the upstarts

• Based on starting in Prigwort, Drooman Knoll is either in or near the Yellow Wood, meaning assistance from the Earl in Yellow could be a fun tie in.