Brittle Stone is given during the task An Epic Request (Shadow Knight) when you reach the update spot in North Ro (northwest corner of zone at 9600, -500).
Make sure you have both SK epic weapons with you.
Make sure you are solo -- no group, no merc.
Clicking the Brittle Stone causes you to be assigned a 1-person task called 'Innoruuk's Curse', and points you southeast to begin the SK-specific script for An Epic Request / An Epic Retelling.