Leonhard is a new NPC in Dark Souls III that you can meet fairly early on in the game. After defeating Vordt of the Boreal Valley, head back to Firelink Shrine to find him leaning on one of the thrones near Ludleth. Talk to him to receive some Cracked Red Eye Orbs. Here, you’ll need to either defeat another player or invader in order to progress. You can either invade someone with the Orbs, wait to be invaded, or just kill the NPC invader at the Undead Settlement.
After you’ve done that, head back and talk to Leonhard and he’ll give you a cell key. Go to the second bonfire at the High Wall of Lothric and take the ladders all the way down until you see the Undead with a halberd. Turn around and open the locked door with the key to find an elevator. This will take you down to a cell with a Darkwraith. Kill the Darkwraith and you’ll get the true Red Eye Orb for unlimited invasions. Go back and talk to Leonhard, and he’ll simply praise you.
You can continue his quest by finding the Rosaria’s Fingers covenant in the Cathedral of the Deep. After finding the covenant, turn around to find Leonhart standing at the entrance. Exhaust his dialogue and leave the area. The next time you come back, you’ll find Rosaria dead. Take the Black Eye Orb. You’ll be able to continue the quest after defeating Aldrich, Devourer of Gods.
After killing Aldrich, take one of the elevators on the left or right to head up to Gwynevere’s chamber. The Black Eye Orb will react. Use it to invade Leonhart and kill him. He will drop Rosaria’s soul, his hat, and his weapon. Here, you can either use Rosaria’s soul to get a boss-level spell from Ludleth, or return it to Rosaria herself.
Be sure to check out our Dark Souls III wiki for more tips and tricks.