1d4 'Random When The Door To Hell Is Left Open' Encounter Table For Your Old School Campaigns

The planes of accursed Atlantis are the subject of demonic and infernal magick all of the time as cults try to gain power over one another and more then one would be wizard or despot makes a pact with powers from beyond the gates of the Abyss or the Outer Darkness. But there are always traces left behind and elements of weirdness that linger after a demonic summoning.

 

After the dust, sulfur, and demonic fringe elements have fled back to the Abyss or Hell by being dismissed by the wizard or magus are there other demonic elements that can linger and exploit the crack or pinprick in reality as broken damn or dike holds back the waters of an ocean? The answer is unfortunately yes as the powers of Hell itself seek to break down the gates of our reality and charge in taking with them all of our existences as one might snuff out a candle. Dungeons and isolated ruins make the perfect venue for such things as summonings where the deeds of darkness will not easily be observed in the wastelands of accursed Atlantis. There is a 40 to 60% chance of these left open doorways developing into full blown Hellmouths

1d4 'Random When The Door
To Hell Is Left Open' Encounter Table

1. A 4 hit point slime made up from the souls of the damned and the fragments of dreams collected by the dross of disappointment pooled into the thing's half existence. Weapons only do half damage as the thing will seek out the nearest wizard to pour itself through every available orifice and open the gateway to the Abyss. Knows 1d3 spells including the open portal spell of seventh level by instinct. 
2. A harpy like half human demonic thing made from the left over bits of various sacrifice victims. The thing will worms and worry its way over to the nearest fighter of first level or greater so that it can make a claw attack for 2d6 points of damage and lay eggs that will develop into horrid imp like creatures  to crack open the doorway between worlds. 
3. The sacrificial dagger has become a living weapon of +2 intent but has been corrupted by the Abyssal forces. The thing has a demonic ego of 30 and intends to murder the next fool to pick it up and use their body as a living sleeve to open the door with the pinprick already here. 1d4 demonic insect like imps already serve this thing. 
4. A winged demon undead thing waits just over the Threshold to pry open the doorway to the abyss for already a demon possessed hag who was the sacrificial victim awaits any fools blundering into this place. She has a bio mechanical spewing weapon that eats flesh as acid with a range of 40 feet and an agenda of her own. Her undead demonic state can be concealed as an illusion and she loves her new state of being and power that comes with it. She will play the victim for as long as it takes to seduce the party into a vulnerable position for sacrifice and murder.