To ensure that the sapling is not recovered and that it twists to the will of Angmar, Cinaid wishes you to eliminate the Free People to ensure that their search for the sapling is for naught.
'The discovery of these shivering little saplings drives the Free Folk and our army into the field of battle. We search for signs of these creatures. We tread where we want and will crush the enemy to collect what we desire. Find the enemy. Stop them from maintaining the natural order.'
Cinaid wants you to make your way into the Ettenmoors and eliminate the Free Folk that wander there looking to assist the saplings. Slaughter them all!
With a good number of free-folk slain, you should return to Gramsfoot and inform Cinaid of your efforts.