Module XL1 Quest for the Heartstone is an adventure for the D&D Expert set, Mentzer/BECMI version.
Let's be honest and upfront right away. This is not a great module. The adventure is widely described as being akin to everyone's first module. The adventure is your basic "retrieve an item at the end of a dungeon crawl" fare.
The real reason behind this module are the toys. Specifically the LJN/AD&D toy line.
In fact you can pretty find an entry for every monster in the toy line, save for Tiamat herself.
Now I am not sure if the module was designed to sell toys (not likely since the markets seemed different to me) or rather as way to bridge the lines. There are references in the module n which toy to use for the encounter and to tell you the truth, it sounds kind of fun.
Reviewing the module again in this light, as an excuse to use the toy line, it actually dawns on me that it would be a blast with the right group.
It should also be said that this module includes the stats for many of the favorite npcs/figures such as Warduke, Kalek and Strongheart, plus a few I didn't even know about.
So viewing the module in this light, is could be quite fun despite it's short comings.