"You enter the hallway. About thirty feet away behind a pit, a goblin jeers at you, pulling out a bow."
"I shoot my longbow at the goblin."
"Your arrow flies towards the goblin, but disappears before it hits."
"I shoot again."
"Your arrow disappears as before. From behind you, an arrow strikes you in the back."
"I try and see where the arrow came from."
"It's one of yours."
The dual portal trap is set up as follows:
Portal (1), open space (O), second portal(2), pit or other hindering terrain a monster can hide on the other side of (X), and then a hostile monster with a ranged attack (M)
Ranged attacks entering either portal disappear, coming out of the opposite portal the next round. Portals release projectiles towards the opposite portal, meaning that ranged attacks can indefinitely move between the portals. The magic of the portals actively aim projectiles at any creatures in the area between the portals. The portals only affect ranged projectiles and magic, and creatures pass through them with no effect. If a monster tries to attack the party from the far end of the trap, their projectiles are caught by the portal, and then used to attack by the opposite portal. (For example, Monster M shoots at the blank space (O). His projectile disappears at portal 2, and then comes out of portal 1, aimed at any creature in the blank space, towards portal 2.)
The portals can be defeated through various methods:
Dispel spells, merely exiting the affected area of the trap, and bypassing it entirely. A group that makes no ranged attacks through the portals is in no danger from their own attacks.