I imagine you want to know how to do it, so here’s a rough guide:
1: You’ll meet him near Lothric’s throne for the first time. When you talk to him he’ll give you 5 Cracked Red Orbs in order to “pillage Embers.”
2: Once you’ve achieved one pale tongue (either through invasion success, killing Darkwraithes or that one in the Undead Settlement) you’ll find him again in the same position. Talk to him and he’ll give you the key to kill that one Darkwraith in the High wall of Lothric which gives you the uncracked Red Eye Orb. He’ll also give you a sly comment about Rosaria in the Cathedral of the Deep.
SIDE NOTE: By now, you’ll have received the covenant, it’s very important to know that wearing this will aggravate Sirris of the Sunless Realms who will leave shrine and you’ll fail her quest. It’s wise to do Leonard’s quest after Sirris’ as there’s no time limit on Leonhard’s quest.
4: In the Cathedral of the Deep, once reaching Rosaria’s room, you’ll need to use one pale tongue to “prove your loyalty” (do this by just pressing “offer pale tongues”) WARNING: this will also aggravate Sirris!!! After resetting the area, Leonhard should be at the door and gives you speech. Exhaust his dialog and leave the area.
5: After resetting the area, Rosaria should be dead and you should receive a Black Eye Orb. To use this Orb, you need to kill Aldrich and go up to Gwynevere’s room, the Black Eye Orb should quiver.
6: Use the Orb to invade Leonard and once you kill him, you’ll receive his gear, his sword and Rosaria’s soul. Use it to either revive Rosaria or transpose with Ludleth to gain a miracle.
SIDE NOTE: Not reviving Rosaria will not stop you from Rebirthing or gaining ranks. This questline will also allow Yellowfinger Heysel to be summoned as an