Does the verb after 'set of' agree with 'set' or the plural noun that follows?

Does the verb after 'a set of + plural noun' agree with 'set' or 'noun'? For example:

'Law is a set of rules that govern/governs society.'

In a main clause where "a set of rules" is the subject, the verb should be singular to agree with "set" (the head of the noun phrase):

A set of rules governs society.

In your example, you build a relative clause that can refer to either "set or "rules". Hence, both are possible with a minute shift in meaning.

Singular, the verb agreeing with "set":

...a set of rules which governs socitey

means

There is a set of rules, and this set governs society.

Plural, the verb agreeing with "rules":

...a set of rules which govern society

means

There is a set of rules, and these rules govern society.