IPA: /hɪəˈrʌpən/
KK: /hɪrˈʌpən/
At this point or moment, often used to indicate something that happens immediately after a previous event or statement.
The committee made their decision, and hereupon, the new rules were implemented.
Hereupon is formed from "here" (from Old English "hēr", meaning in this place) and "upon" (from Old English "uppon", meaning on or upon). The word "hereupon" means on or upon this place or matter.
Think of being 'in this place' ('here') and then adding 'on' or 'upon' — that's how 'hereupon' means something that is on this matter or place.