If you are refering to the taming quest I think you need a specific nightsabre. It should be written in the quest details. U can open that by pressing 'L'. Good luck
EDIT:
Found it. You have to tame a Nightsaber Stalker.
If you have trouble with quests in the future check http://www.thottbot.com/?f=q for details.
Edited, Aug 16th 2007 3:48:39pm by Holingar