There are specific methods to trigger Beyond encounters in Path of Exile:
When a monster is killed in a Beyond-enabled instance, there is a 15% chance for a small red orb to appear on the ground. If four of these appear close to each other (roughly one screen), portals will open, spawning Beyond demons.
Once a substantial number of Beyond demons have spawned in the instance, each pack spawned grants a stacking chance for the next pack in that area to be replaced by a boss. Only one Beyond boss can spawn per instance. Once a boss has spawned, Beyond portals will no longer spawn in that instance.
Atlas Passive Skills: Investing in specific Atlas Passive Skills on the Atlas tree can grant a chance for maps to attract Beyond encounters.
Beyond Scarabs: Using consumable Beyond Scarabs on the Map Device guarantees a Beyond encounter in the activated map.
Map Device Crafting: The Map Device itself offers crafting options to imbue maps with the Beyond modifier, forcing a Beyond encounter within that map.
Certain nodes in Delve (Azurite Mine) also contain Beyond encounters.
The legacy map mod Otherworldly(Slaying Enemies close together can attract monsters from Beyond this realm) also adds Beyond demons to the map instance.
Demons are divided into three factions, and only one faction will spawn in any single instance. These factions are the Pale Scourge, the Flesh Scourge, and the Demonic Scourge.
Drops
Tainted Currency: Beyond demons have a chance to drop Tainted Currency, a special type of currency used for crafting powerful items. Defeating Beyond bosses guarantees multiple Tainted Currency drops.
Kirac Map Mods
Name
Kirac’s league modifiers
Cost
Beyond
Slaying Enemies close together can attract monsters from Beyond this realm.