Obviously, with this achievement, devs wanted you to feel like Prometheus. Well, you just have to light a fire between two NPCs, literally – not The Doors way , while they argue about it (typical human behavior, innit?). There are a few locations where they can spawn in every zone, except 1-5 zones, Coldharbour and Cyrodiil, all next to main roads. Also, they disappear 1-2 minutes after you interact with them. Those reasons make it really hard to complete this achievement, while many people are online and playing. The witching hour seems like the best option to find them, if you are not afraid.
A Bandit’s Heart and Golden Palms achievements share locations with this one, thus, traveling around using main roads (not confirmed, but it looks like they only spawn near them) can get you 3 in 1! Sometimes, they even share the exact same location, and you just have to relog and interact with new NPCs related to other achievements.
This guide is work in progress, thus, if you know any location not listed in the gallery below, please share it with us and ESO community.