When you're ready to fight, go speak to "Wyrmbait". He'll "fetch" her. Once Icestorm is close enough we'll chain her to the ground and finish her off! Hopefully, with your help, we'll be able to take her down and get you that phylactery.
Once you've got the phylactery, return to Wintergarde Keep and report to High Commander Halford Wyrmbane. He's been making preparations for the final showdown with Thel'zan.
Rewards
You will be allowed to choose one of these rewards
We've lost so many good soldiers to that bastard, Thel'zan. I wish I could be there to see you crush his phylactery in front him and end his existence.
Notes
Talk to "Wyrmbait" when ready. Stay back near the harpoon launchers and wait for Icestorm to get chained down, then nuke her from range. The friendly NPCs should help kill her. If that's not feasible, she can be attacked from melee range as well, but be warned that she will hit back. Consider using a cooldown or two to speed up the fight. Don't worry about tagging her, as she'll drop the phylactery on the ground. Loot it and head back to Wintergarde.