Expresses a strong logical connection: 'now that A is the case, naturally B follows'. Often implies a sense of obligation, determination, or natural consequence. Used with verbs and adjectives in the plain form.
Now that I've promised, I will definitely keep it.
Since I've come to Japan, I want to climb Mt. Fuji.
から is a plain, general causal conjunction meaning 'because', while からには implies that once a condition is established, a determined or obligatory outcome naturally follows.
Compound of the case particle から ('from') and the particle には, which together grammaticalized into a conjunctive expression meaning 'now that' or 'since'. The exact historical development is uncertain, but it is a natural extension of the source-marking function of から.