Return to Firelink Shrine and speak with the Shrine Handmaiden to notice a new item called a Budding Green Blossom, which has a description that informs you that Sirris has headed to the Pit of Hollows.
Warp to the Dilapidated Bridge in Undead Settlement and make your way to the Curse-rotted Greatwood’s boss arena to find Sirri’s summon sign.
You’ll be summoned to a fight with her and her grandfather, Holy Knight Hodrick, and you must not let Sirris die or she will be gone for good. Fighting Hodrick is similar to when he invaded you at the beginning of the game, but you’ll want to keep his attention on you and not Sirris.
Attack anytime he changes his focus to Sirris with a long combo and back off and let Sirris stun him before you attack when his attention is on you. If at any point Sirris is wounded pretty bad you’ll want to go all out, even if that means death from lack of health, because you can always return to her world again.
Once Holy Knight Hodrick is dead, return to Firelink Shrine and speak with Sirris to accept her loyalty and then travel to the Pit of Hollows bonfire to collect the Sunset Armor Set near the Mound-makers altar.
You can now return to Firelink Shrine and exit the arch on the second level, follow the cliff on the left side to the area with the Grave Warden overlooking the edge, and pick up the Sunset Shield from the tombstone.
Note: While this is not the end of Sirri’s quest, the only other item obtained is the item she drops upon death. This means that the only reason to keep her alive would be for aid in future boss battles as a White Phantom.
For this reason we’ll now progress with Ringfinger Leonhard’s quest and the Rosaria’s Fingers covenant.
Travel to Rosaria’s Bed Chamber in Cathedral of the Deep and finally join the Rosaria’s Fingers covenant, making sure to offer her at least one Pale Tongue.
Doing this will cause Ringfinger Leonhard to move to this area, where you can speak with him, and also force Sirris to appear in Firelink Shrine, where you must kill her to obtain the Sunless Talisman .
Return to Rosaria’s Bed Chamber and collect the Black Eye Orb from Rosaria’s corpse and then make your way to Gwynevere’s chamber at the top of Anor Londo’s boss room. Here you can use the orb to invade Leonhard’s world and defeat him for the Soul of Rosaria , Crescent Moon Sword , and Silver Mask .
Note: The Soul of Rosaria can be used to revive Rosaria, but this is pointless since you can give covenant items without her being alive and the soul can be transposed into the Bountiful Sunlight spell.