Leonhard Dark Souls III Guide
Last update: 11 May 2016
Firelink Shrine
Leonhard can be found standing next to the tallest throne after you enter the shrine. During the first conversation you will receive 5x Cracked Red Eye Orb from him. After the second conversation you will get the Lift Chamber Key, but only if you first find a Pale Tongue (for example while helping Siegward in the Undead Settlement) or kill the Curse-Rotted Greatwood.
The door that can be opened with this key can be found in the same tower as the Tower on the Wall bonfire in the High Wall of Lothric location (in this chapter it is the High Wall of Lothric - from the Bonfire chapter, paragraph Locked door in a tower with Tower on the Wall bonfire).
After obtaining the Red Eye Orb for defeating the opponent at the bottom of the tower, go back to Leonhard. You will receive the Applause gesture from him.
If you join Rosaria's Fingers and give her at least one Pale Tongue, then Leonhard will move to her chamber.
Note: Joining the above mentioned alliance will make Sirris your enemy and you won't be able to finish her quest.
Anor Londo
If you are a member of Rosaria's Fingers and you give her, at least, one Pale Tongue, return to the bed chamber after the fight with Aldrich. You find Leonhard there. Reload the location and you will find Rosaria dead (you can still take advantage of her "services", though). Collect the Black Eye Orb in front of her and go to Anor Londo, do where you fight Aldrich. To the right of the bonfire, there is a passage with an elevator behind it. You can take the elevator up. In a small room, there is Sun Princess Ring. You can also use the orb here that you have found to travel to the world of Rosaria's killer - Leonhard.
Be aggressive while fighting him, but watch out while pounding at the enemy's shield because he can parry your attack and counter you. Apart from that, pay attention to Leonhard's spells because he casts them sometimes. Try to push the opponent towards the wall and have him heal himself. After you notice him doing that, keep attacking, which should lead to his death. For the battle, you will be rewarded with Rosaria's Soul (you can use it to bring her back to life or trade with Ludleth), Moon Crescent Sword and Silver Mask.