This quest is not available in game.
Travel to Tyr's Hand, southeast of the Marris Stead. Once there, take a bite of the Rotten Apple. While under the guise of the Scarlet Crusade, deliver the Grand Crusader's Command to Crusader Lord Valdelmar.
I have made some adjustments to this command. Should our little ruse work, they will expose the Scarlet Oracle and we shall strike!
Now pay attention, <class>. You are to deliver this command to the Crusader Lord, Valdelmar.
How? You are going to hand it to him, imbecile.
Take the command and this rotten apple to Tyr's Hand. When you cross into the city, eat the apple. You will transform into something more... palatable to the humans. Take the command and hand it to the Crusader Lord, Valdelmar.