What quest is the Umbra sword involved in in Oblivion

The shrine quest of Clavicus Vile. You must kill the bearer of Umbra before you can take it. Clavicus also wants you to return Umbra to him, in exchange for his helmet 'fit for a God', "The Helmet of Clavicus Vile". You can, however, refuse his offer and take Umbra, leaving behind a very unhappy Clavicus.