The Siren converts all neutral/enemy units (civilian and military) to your villagers within the radius. They now belong to the town that cast the miracle on them. You have to be careful though since casting this could mean you suddenly have 200 extra villagers, and suddenly you cannot house or feed that many. When converting enemy soldiers, they turn into regular villagers.
You can use an epic like this anywhere except in the enemies' influence. The siren will still affect villagers within the enemies' influence, you just can't place ground zero in the influence. For this miracle, it hardly matters since the radius is so huge. Casting on the outskirts of the town is enough to swallow half the population. I just did this on Land 5. The Japanese had 300 villagers and another 300 soldiers. One cast and he had 60 of each with the rest now belonging to my primary town.
Converted people will make their way to your town. More importantly, they're included in your population immediately.