testament vs legal instrument

testament and legal instrument both are nouns.

Word	Noun	Adjective	Verb	Adverb
testament	Yes	No	No	No
legal instrument	Yes	No	No	No

As nouns, legal instrument is a hypernym of testament; that is, legal instrument is a word with a broader meaning than testament:

testament: a legal document declaring a person's wishes regarding the disposal of their property when they die
legal instrument: (law) a document that states some contractual relationship or grants some right

Other hypernyms of testament include instrument, legal document, official document.

testament (noun)	legal instrument (noun)
a profession of belief	(law) a document that states some contractual relationship or grants some right
a legal document declaring a person's wishes regarding the disposal of their property when they die
strong evidence for something