After choosing an option (either kneeling to, standing before, speaking to or calling Saradomin a snow imp), progress through the conversation.
If you have openly declared allegiance to a god through a God Emissary, you can bow before him if he is your chosen god, or if you have sided with any other emissary, initially defy his request and declare your loyalties to another god. Since you will need a disguise to enter the fortress, take the armour just south-west of you, made up of the Black Knight captain's helm, cuirass, gown, gauntlets and boots. He'll tell you that you'll be impersonating Captain Dulcin, the overseer of the Black Knights. With your permission, Saradomin will teleport you to the entrance of the Black Knights' Fortress.
Lieutenant Graves will ask to guard Sir Owen, but before he can do anything, Owen will incapacitate him. You're given a choice whether to kill Graves or tie him up.
The Death of Chivalry is a quest that replaced Black Knights' Fortress.
It features Sir Owen and Saradomin and is the second quest to take place in the Sixth Age.
Pick either option and head to Captain Dulcin's quarters.
Search Dulcin's desk in the north-eastern area to find Captain Dulcin's journal, which you're unable to unlock. Search the wardrobe for a Letter from Lord Daquarius, search the box of candles for three red candles, search the shelf for chalk, and then take the Grimoire.
Now read the Grimoire to learn the ritual and gain 250 Magic experience.