A more thorough explanation:

Codicillary is an adjective that describes something related to a codicil, which is a legal document that modifies or adds to an existing will.

• My grandfather's codicillary instructions changed the beneficiaries of his estate.
• The lawyer drafted a codicillary agreement to clarify the distribution of assets.

These examples show how the term codicillary is used to describe something that is related to a codicil. In the first example, the codicillary instructions modified the original will by changing the beneficiaries. In the second example, the codicillary agreement added new information to the existing will to clarify the distribution of assets.