Beyond Portal Mechanics (my take)

Soooo, just wanted to put this out there b/c I couldn't really find much myself. Here's how I think it works: each mob has a chance to drop a portal, but I don't think it's related to how big the mob is; once four of these portals are together, the first round of demons spawn. Once these spawn, you'll notice bigger portals around these demons which go away if you kill them. As such, to get the next round to spawn, you have to leave them alive and kill other mobs around the bigger ones. To get one of the bosses to spawn, you need 3 - 4 of the regular spawns in the same area. Keep in mind that the portals do not go away, though, and you can kite monsters back to it for more chances. Doing this, I have drawn out Na'em and Bameth. Hope this helps, and if somebody thinks I'm horribly wrong, feel free to share with the group!

I agree on most parts. Chance to spawn portal, regardless of size/hp pool etc., requires 4 portals to spawn demons. Although I do not think you have to leave the spawn alive. It has happened multiple times to me that the spawn itself spawned another portal. I think they just count as regular monsters.

You do not need to keep any of the beyond mobs alive to spawn the boss. You just need 4 maws close together.