Make your way to the first Drangleic Castle bonfire, then take a left and go through the door at the end of this section. Now head downstairs to enter the Throne of Want itself, then walk down the path until you reach a misty door. Make sure you're prepared to fight before heading through this door, as you're about to face an epic battle against the Throne Watcher and the Throne Defender.
How to kill the Throne Watcher and Throne Defender
Fighting both the Throne Watcher and Defender at the same time is very tricky. The Watcher is a pretty agile opponent, while the Defender is slower but packs a meaner punch. Try to keep both of your opponents in view at all times, and attack whichever of the two is nearest to you.
If you can kill the Watcher first, then by all means do so, but just keep in mind that you need to kill both of them at around the same time. If you don't kill them both within the same tight window, the one you've downed will come back to life while you're finishing off the other one. Do try to kill the Watcher as a priority, but just make sure both health bars are being whittled down as evenly as possible.
You can expect both of these bosses to come at you with a fairly standard two to three-hit combo attack when they're in melee range, and a lunging attack from distance. The general strategy you need to employ is to keep them far enough from each other so you can counter-attack one without worrying about the other getting stuck into you while you're doing it. Use a fast, ranged attack if you can, or make use of a spear-like weapon so you can shield-block while attacking.
Strafe around the enemies in a big circle, keeping them in view and getting your own damage in whenever possible. Wait for openings to present themselves before doing so - never force the opportunity - and attack once or twice depending on the speed of your weapon. Patience is absolutely key to winning this fight, so don't be tempted to take any short-cuts as the battle drags on.
Once you've killed both of your opponents, talk to Shanalotte who's right by the Throne of Want entrance. Now head down the long path ahead of you and go through the misty door for the final showdown against Nashandra.
You can find the rest of our Dark Souls 2 guide from the first page.