Runestones are gray slates with a colored rune inscribed upon them. They are necessary for entering the Underworld, where Skorne's home is. Collecting Runestones is a key objective, much like obtaining Shards, activating Obelisks, and finding Crystals. The Runestones are hidden throughout the Realms, and heroes will need to search high and low for them. Should a hero leave an area without obtaining the Runestone within, they'll be subjected to Skorne's scornful laugh.
There are only 12 Runestones here. Every future adventure would have a total of 13.