‘Till death do us part.

It is supposed to be: ‘Till death us do part, a contraction of: until death us do depart.

An older version of the final phrase is “and to obey, until death us do depart” where “depart” means “separate”. “Until death us do depart” had to be changed due to changes in the usage of “depart” in the Prayer Book of 1662. In the 1928 prayer book (not authorised) and in editions of the 1662 prayer book printed thereafter “and to obey” was retained (in the 1928 book an alternative version omitted this).