Battle for Windhelm

Information
This quest belongs to The Imperial Legion quest line.

Quick Walkthrough

 * 1) Go to Rikke and heed her command of meeting the soldiers gathering for the attack.
 * 2) Enter Windhelm together with Tullius, Rikke and the imperial soldiers.
 * 3) Make your way through the city towards the Palace of Kings, killing all Stormcloaks you encounter.
 * 4) After entering the palace, defeat Ulfric and Galmar Stone-Fist.
 * 5) Then if you want to, kill Ulfric. Or let Tullius have the honor.
 * 6) Report to Tullius.

Detailed Walkthrough
When you report back to Rikke after "The Battle for Fort Amol" quest, she will give you a small amount of gold. Right after that, if you stand near her, she will bellow: "We're taking the city, meet the soldiers gathering for the attack. Move it!"

Go to Windhelm and join Tullius and his soldier outside the gate of Windhelm. After he has given a short speech, everyone will storm through the gates and the last battle will start.

After entering the city, make your way through it, killing all Stormcloak soldiers in your way. Most side streets are blocked. Just follow the path, where soldiers are coming in the opposite direction, and at the end you will find the Palace of Kings.

Enter the palace and face Ulfric together with Rikke and Tullius. After defeating Ulfric and his right hand Galmar Stone-Fist, you will be given the choice of either killing Ulfric or letting Tullius do the deed. Either way, you will be given Tullius sword.

After Ulfric is dead, speak with Tullius and the quest "Battle for Windhelm" as well as the "Reunification of Skyrim" will be completed, ending the Imperial quest line.