Guardian Dragon

The King's Ring opens three gates:

1. The gate to Aldia's Keep. Aldia's Keep is on the path to Dragon Shrine, where the Ancient Dragon is located. Exhausting the dialogue of the Ancient Dragon would grant you the Ashen Mist Heart, an item required to enter the arena of the Giant Lord.
2. The gate to The Place Unbeknownst bonfire. This bonfire is next to the arena of the Giant Lord. Defeating the Giant Lord would grant you the Giant's Kinship, which unlocks the final boss of the game.
3. The gate to the arena of the last two bosses. The last two bosses are found in the same arena. However, you require the Giant's Kinship for the final boss to spawn.

You must collect the Ashen Mist Heart and the Giant's Kinship in order to unlock the final boss of the game. The first order of business is Aldia's Keep.

Speedrun through Aldia's Keep NG+7, Dark Souls 2 SotFS

Aldia's Keep

Start from the Ruined Fork Road bonfire. Three paths are spread out ahead of you. The middle path leads to Aldia's Keep. Follow the path until you encounter a Royal Guard standing in front of an iron gate. Defeat the Royal Guard, then equip the King's Ring to open the iron gate. Enter the gate and light the Foregarden bonfire. Rest at the bonfire as necessary. Attune any spells you need for the upcoming boss fight. Crystal Soul Spear is particularly useful against the boss.

Ascend two short flights of stairs to enter Aldia's Keep. Before entering Aldia's Keep, you might want to collect a Radiant Lifegem in the pool of water and the Twilight Herb to the left of the entrance. As soon as you enter Aldia's Keep, Aslatiel of Mirrah would invade you. The red phantom is easy to defeat. Simply spam Crystal Soul Spear. Alternatively, you might want to sprint to the third level and enter the room behind the Giant Basilisk to despawn the NPC invader. Pull the lever hanging from the iron dragon and a gate should open, leading you to the area boss. Sprint to the end of the hallway until you reach a gate. Ignore all enemies. An Ogre is standing behind the gate. Do not stand in front of the gate, otherwise the Ogre would smash the gate and hit your character. Run to the right side of the gate, wait for the Ogre to break the gate, then dash to the second gate, and open the second gate. Now sprint to the third gate. The third gate has the same ambush; an Ogre is standing behind the gate. Position your character to the right of the gate. A Twilight Herb is found nearby. Grab the item as necessary. Wait for the Ogre to break the gate, then dash to the fog wall. As long as your character is near the fog wall, enemies would not approach you. Take the time to do any last minute preparation.

Guardian Dragon

Sorcerer versus Guardian Dragon at Aldia's Keep NG+7, Dark Souls 2 SotFS

Enter the mist and immediately lock onto the boss. Cast Crystal Soul Spear, while moving your character to the left. Perform another two casts of Crystal Soul Spear, while moving your character to the left again. You should be able to cast Crystal Soul Spear at least three times before the boss can fight you. The boss would usually perform either of two attacks.

1. Swoop upward and breathe a fireball at your character. Sprint to the left, wait for the fireball to hit the ground, then cast Crystal Soul Spear.
2. Stay on the ground and breathe fire at your character. This attack can be tricky because the fire can either sweep out an arc from your left to your right, or the fire can be aimed straight at your character. In any case, move or roll to your right to avoid the fire. If the fire is aimed straight at your character, move or roll out of the range of the fire and cast Crystal Soul Spear. You should be able to cast at least three times before the boss stops breathing fire.

Another attack to be wary of is when the boss swoops upward and towards your character, then breathe fire downward. Stay underneath the boss and cast Crystal Soul Spear. Be careful of the fire. You want to position your character such that you can see the boss' tail. As you cast Crystal Soul Spear, your character moves a little bit forward and away from the fire.

Defeat the Guardian Dragon and ride the elevator up to Dragon Aerie. Find a nearby iron ladder, climb up, and look for a nearby zip line. Take the zip line until your character is dropped near a bridge. Cross the bridge and light the Shrine Entrance bonfire. Rest as necessary.