Union sits next to 4 phonetic neighbors — Orion, Onyx, and Owen are the closest matches by sound. Most share Union's Latin unio (oneness)-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"the act of joining together, unity, oneness"
"serpent-like, winding"
"God will add, God will increase"
"from Jerez (the wine region)"
"night, dark beauty (beautiful night)"
"from the round hill"
"plural of ovum (biological)"
Similar sound: These names share phonetic patterns with Union — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Latin unio (oneness) origins, giving them the same cultural heritage as Union.
Rhythm: Union is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.