Death is the subject in the phrase. It's from a 16th century Anglican prayer book, The Book of Common Prayer, in which it was "till death us depart," with death being that which would depart (separate) the people making the vow ("us"). However, something that was more common in the 16th century (and is rather more rare in English now though many common phrases still use it), is the subjunctive mood, in which conjugation of verbs has a different form (usually the bare form).
Death is the subject in the phrase. It's from a 16th century Anglican prayer book, The Book of Common Prayer, in which it was "till death us depart," with death being that which would depart (separate) the people making the vow ("us"). However, something that was more common in the 16th century (and is rather more rare in English now though many common phrases still use it), is the subjunctive mood, in which conjugation of verbs has a different form (usually the bare form).