In Harry Potter and the Half-Blood Prince what is one thing Sererus Snape agreed to do in Unbreakable Vow?

In Harry Potter and the Half-Blood Prince, Severus Snape agreed to protect Draco Malfoy's mother, Narcissa Malfoy, in an Unbreakable Vow.

This vow was the result of a deal made between Snape and Lucius Malfoy, where Snape would help Draco carry out a dark task (which was later revealed to be killing Dumbledore). In return, Lucius ensured Snape would be able to protect Narcissa from any harm.