There's a Banished Knight holding a shield and patrolling around the building; you can kill it then rest at the Site of Grace inside to make it appear again. Repeat the process until it drops the ...

If you take a look at the acqueduct, you'll see the Crucible Knight holding a shield (which is the one you must kill) patrolling the zone. If you are a caster, you can kill it from there as you'll ...