Demonic Portal

Uttering foul words in Abyssal you pour forth the blood and open a one way portal to a random layer of the Abyss. The spell does not specifically summon any demons, so much as it provides a portal which demons can use to enter the same plane as the caster. 

When the spell is cast the portal will stay open for 1 minute, during which time any number of demonic creatures may come out. Any creatures that come out of the portal are returned back to the Abyss when they are reduced to 0 Hit Points, or the spell's duration ends. The caster has no ability to end the spell early, or close the portal, and is not granted any control or protection from what comes out of the portal. Once the portal is opened, if the caster and any other nearby creatures are not already in combat they should now roll initiative.

At the start of every round roll to determine if anything comes out of the portal. If one or more demons do arrive, they roll initiative and will go that round on their turn. 

While the portal is open there is a chance that something will come out each round until the portal closes. At the start of each round while the portal is open roll 1d6: 1-3: Nothing comes out this round, 4-5: Lest demons, 6: Random demons.

Least demons: roll 1d6 again: 1-2: 8 Manes, 3-4: 4 Dretch, 5-6: 1 Quasit.

Random demons table:

None of the demons that come through the portal are under any compulsion to obey the commands of the caster or anyone else, and will act according to their nature, which is generally chaotic, murderous, and destructive. 

The portal can be closed early by casting the Banishment spell on the portal.