Monster Manual classification: Unknown
Attack: Summon Fire Reavers
Special Qualities: DR /-, Immunity to all elemental damage and sonic. Alignment, Light, Force and Negative Energy damage works normally.
Description: While those portals are active, reinforcements will keep spawning infinitely, until everyone is dead, the demon is defeated, or everyone teleports out (using Dimension Door or recalling).
A good archer focused on "portal duty" can take them down before it can summon reavers on heroic.