The Lich’s Heart is a free adventure by John Thompson for 10th-level characters. It has a very short and sweet blurb on the DMs Guild:
This is the adventure I wrote to propose to my girlfriend.
It reads like an old-school adventure. The adventurers explore a ruined castle, encountering various tricks, traps and monsters. Some puzzles require items from other areas of the castle to solve. I very much like the puzzles involving books; they’re very elegant and satisfying.
Nothing is too complex; the rooms have descriptions to give them flavour, but there isn’t an abundance of details. Monsters are simply described and most lack motivations.
The maps are hand-drawn on graph paper; the presentation is very basic and, occasionally, the columns run into each other. There are numerous errors that could have been fixed with proofreading. It is, by no means, a professional product.
However, I found it a charming adventure. It hearkens back to the early days of D&D, and it includes enough surprises to entertain me. The ending is perfect. Recommended.