Tanval Indoril is inside the Tormented Spire working to unleash the Brother of Strife. Tanval's Vanguard holds the approach. If we don't stop Tanval, he'll unleash Sadal and destroy Stonefalls.
Kill Flamespinner and collect her key
The key to lowering the final ward keepign us from Sadal lies in the grasp of a powerful Daedra named Flamespinner. I must slay her and claim the key.
— game journal
Use the summoning bowl on the spire to summon the Daedra.