Some of the descendents of «advenio» according to Wiktionary:
English: advene, advent, adventure
Franco-Provençal: avegnir, avindre
Old French: avenir, aveindre. avenir surviving only as a noun in Modern French.
Portuguese: advir, avir
Spanish: advenir, avenir.
Avenir and aveindre are rather Old French than French.
Only advenir (ad+verb. venir) still exist. It means arriving in the future (figuratively), to happen.
Linked with Latin adventura (from advenire)= what is likely to happen (to arrive).
Adventure is also related. From French Aventure (probably also adventure, as the English kept the form).
"Adventus" was rather a Roman ceremony in Roman times.
The Christian "adventus" seems to come later. (I don't know exactly when)
The adventus was a ceremony in ancient Rome, in which an emperor was formally welcomed into a city either during a progress or after a military campaign, often (but not always) Rome. The term is also used to refer to artistic depictions (usually in relief sculpture, including coins) of such ceremonies. Its 'opposite' is the profectio.
The profectio ("setting forth") was the ceremonial departure of a consul in his guise as a general in Republican Rome, and of an emperor during the Imperial era. It was a conventional scene for relief sculpture and imperial coinage. The return was the reditus and the ceremonial reentry the adventus.
It remind me when our President enters a city...