Here's what you need to know about it:
* Location: It's found deep within the Depths area. You'll need to fight your way through several enemies and navigate dangerous terrain to reach it.
* Use: The Shard of Blade Stone is used by the Blacksmith in the Nexus to upgrade weapons to +10. This is the highest level of upgrade for weapons in Demon's Souls.
* Rarity: It's a very rare item, and you can only find one per playthrough. This makes it valuable for upgrading your most powerful weapon.
* Note: You need to have already upgraded the weapon to +9 using the Blade Stone before you can use the Shard of Blade Stone.
If you're struggling to find it, there are plenty of online guides that can help you locate it.