Trying to find a series I read in the 90s about a boy who finds other world and overhears plot

Question

He’s visiting a house, I think his uncle's, for the summer. He finds a way to another world in a chest? He finds himself in a room. While there he finds other world creatures and overhears them talking about taking over/destroying our world. I was super creeped out by it and I can’t find it anywhere. I know there’s at least one sequel because it isn’t resolved in this book. Any help is appreciated, I’ve been looking for years! I think there’s mist involved as well, which might have helped creep me out. Thank you!

Answer 1

Might it be William Sleator's The Boxes?

Orphan Anne Levi tolerates her distant Aunt Ruth, with whom she lives, but adores her mysterious Uncle Marco, who flits in and out of their lives at irregular intervals. When he gives Anne two unusual boxes with strict instructions not to open them, curiosity gets the better of her. Opening the first one, she releases an unusual crablike creature that grows and reproduces rapidly; the life form and its offspring construct a fantastic palace in the basement and communicate with Anne telepathically.

Dismayed by what she has done, Anne opens the second box, which she had hidden in her closet, revealing a clocklike object that has the ability to slow down time at the basement creatures' request, but only when Anne agrees to carry messages between the creatures and the clock.

Unfortunately, the owners of a suspicious development company are intrigued by the time slowdowns and increase their ominous efforts to control Anne, her home, and the strange devices within it.

Unfortunately, while the book ends with a hook for a sequel, one was never written, although there was a prequel, Marco's Millions.

Answer 2

I found the answer with the help of Reddit, after DECADES of looking. It’s The Mansion in the Mist by John Bellairs

Anthony Monday is delighted when Miss Eells and her brother Emerson invite him to spend a restful summer vacation in an old house on a desolate island. But soon Anthony finds a chest that can transport them to another world -- a horrifying place where a maniacal group is plotting the destruction of the people of Earth. Will Anthony and his friends be able to save mankind -- or will they perish in their desperate struggle?