Yaniel was a bright-eyed paladin who made frequent forays into the worldwound atop her mighty steed and but for a few tumbles with an aasimar paladin she was focused on her work. Her focus caused her to ignore certain signs of pregnancy until it was too late and she had been snatched by Abyssal forces and granted to the Invidiak in 4694. He soon discovered her pregnancy and took great delight in wearing her while describing the symptoms of her pregnancy and the guarantee of her child's soon-to-be warped nature.
Although she knew that her child would be tainted, Yaniel could not help but call out to a gentler goddess for assistance in granting one wish. She prayed to Shelyn to ensure that her child would know her love, no matter what would happen later, for the child was blameless in its affliction. Shelyn could no more get involved in rescuing Yaniel and her child from their fate than Iomedae but this wish she could grant.
While Yaniel survived the birth, she did not survive long for soon after she was taken by another demoness and cocooned so that the demon could wear her likeness. Her child, on the other hand, was passed into the communal nursery with all of the others. There was some concern that he would consume the other children but none would deny the Invidiak's wishes (at least, none who would care).
The child's celestial blood would normally have been entirely extinguished but for the ever present love he felt from his mother. Instead it interwove with the Abyssal infusion of essence just enough to allow the son a tiny crack, a mere sliver, of human conscience. This was never enough to make him a good person, or even neutral, but it allowed him to have human attachments fostered by his understanding of a mother's love for her child. This makes him redeemable with the same strength of chaotic evil alignment as would be found in tieflings on the nastier side of the band.
It is now 4713. The child, Willem, is now seventeen, though he has aged more slowly and therefore has the mind, build and maturity of a thirteen-year-old. Rather than being treated as impressive or special, he has lived his life much like any other tiefling - albeit one who was more dangerous to cross. He is a 1st level expert who spent much of his life in the mines being treated as simple by the other humans who didn't realise that he was simply aging more slowly.
Once the PCs' army finish taking apart Drezen's defences, Willem will approach them from the cover of the one of the buildings and offer information on the whereabouts of the two nursery and mine/school. Willem doesn't fancy his chances out in the open with them, presuming he'll be destroyed, and will attempt to flee out the back window and fly away if they approach him.
Any attempts to Sense Motive (DC 15) him will reveal that he isn't leading them into a trap, that he fears for his own safety and that he doesn't trust them an inch with his own life but believes that the children have a better chance with paladins (whom he assumes will execute them) than with the cultists (whom he assumes will ritually slaughter them in agonising rites of apology to their demonic masters).
Attempts to Intimidate him into surrendering will automatically fail unless they can surround / trap him (which is hard to do with his incorporeal options, see "Half-Invidiak" template in Demons Revisited). Attempts to Bluff him into giving further information as a distraction while others circle around behind him may work (his Sense Motive is +6) but only for 1d4+2 rounds (enough for a single question and answer). Attempts to use Diplomacy into gathering his trust is particularly hard (DC 35 as he is hostile / terrified for his life). It is easier to use Diplomacy on him once he is captured as by that point he will feel that there is no point fighting them.
If they attack him, they will likely immediately kill him with an attack or two so do give them a Sense Motive check to determine his motivations especially once they see what he truly looks like. It's hard to confuse a half-demon with a tiefling, after all.
Capturing him alive will net them 1d6 goods and 1d4 potions as he tries to curry their favour. He will remain with them for a single night before whisperings among the paladins spook him into trying to flee again. He won't run if they win over Mr. Fiprac (located in the school) as he trusts the man as a sort of father figure. Instead he will want a sword and the chance to join them in an attack against his father. He won't survive a delve into the basement so it's best they leave him behind.
Due to the risks of raising children around surly, hostile and aggressive demons, the humanoid and tiefling armies have taken to hiding their children away within two underground nurseries built into basements -- halving the chance of a bored demon choosing to murder all of the cultists children at once. The children's tiefling lineage is the only thing that protects them from rickets due to lack of sunlight and the clerics that look after them are harsh but not particularly abusive for fear of angering the various parents of such children. After all, only the children of important cultists are raised in relative safety here rather than with their families.
1. Deskari Temple Entrance: Five lvl 3 clerics (see Scholar Priest, NPC Codex) kneel on red cushions around a bowl of haemophagic locusts placed before a carved idol of Deskari. They frequently make offerings of blood from their own wrists to the swarm. The locust swarm will remain in its bowl unless the intruders attack. While diplomacy is an option, it will only work on a DC 30 roll.
2. Living Area: This area simply has a hearthfire, table and two bench seats capable of seating three people each. There are cooking utensils and a small open scullery and pantry beside the hearthfire. The pantry contains enough supplies to keep the toddlers and clerics fed for a week. Should the PCs choose a genocidal answer to these children, they can gain 3 points of consumable food for their armies instead.
3. Bedroom: There are three bunk beds in this room as well as an elaborate stone altar to Deskari with a few old blood stains upon it. Depending on your player's sensibilities (i.e. trigger warning), you might include a flip up lid revealing a chamber full of flesh eating beetles with a stack of oddly shaped babies' skulls and bones forming a decorative mural on one wall. The clerics ritualistically murder children who have some visible disability.
4. Nursery: Thirteen tiefling babies lie in make-shift cots with a range of different tiefling aspects - including the odd qlippoth marking due to certain primal abyssal energies rather than bred heritage. Three bottles of breast milk sit on the dresser. These self-cleaning conjuration bottles can each provide sustenance to a baby seven times per day.
5. Playroom: Eight toddlers play together with makeshift toys on dirty bedding. There is a cage in this room for disobedient toddlers. All are tieflings due to the contamination of the Worldwound.
NOTE: These tiefling babies and toddlers are all Chaotic Neutral though they are by no means easy to raise (i.e. babies are demanding, frequently cry for no real reason, rarely sleep through the night; toddlers are destructive, stubborn and throw more tantrums than normal).
These clerics are less considerate of their charges. All but one fully enjoy inflicting unfair punishments for random irritations to keep the children on their toes as any claimed child would have been returned to their parents for training at this point. Once a tiefling child turns approximately five years old they are sent to 'school'. There is a miniature celebration for the new attendees who are fed cake and apple juice and allowed to play in the front room. They're even allowed a night's stay in real beds (otherwise occupied by the cultist slave drivers).
The following morning, though, the previous day's lies are ripped away and they are bullied mercilessly into the mines. Once the kids hit puberty they are drafted into the army (if needed) or left to work the mines. The half-invidiak, Willem, was an exception to this because he chose to remain in the mines despite the ill-treatment of others, finding safety and succor in the darkness.
Only one individual, Mr. Fiprac, has been good to the children. Mr. Fiprac is a true neutral level 8 elf ranger (see Orc Slayer in NPC codex but amend favoured enemy to humans). During a sortie into the wildlands, he was startled by a fiendish bear and fell into a crevasse during a sandstorm, breaking his leg. Against all odds he managed to crawl / hobble back to Drezen but found the demons less than friendly, barely managing to hide in the school mine.
Although he is a capable ranger, Mr. Fiprac lacked friends among the cultists and therefore couldn't get his leg injury healed. Since demons have little patience with those who show weakness around them, he knew that leaving the school for long would be a short route to death. He managed to retain his position in the mine as a teacher, using his obvious patience and guile to successfully ensure the school made its quote of distance per day, even through bedrock. This proved vital to the cultists as failure to meet requirements meant a cultist would lose a limb and two of the kids.
When the invasion happened, Mr. Fiprac faked the children's escape and instead hid them in the furthest section of the tunnel system. They managed to roll two barrels of water with them when the fighting broke out but are now desperately hungry but too scared to leave in case the cultists sacrifice them to please Deskari.
The cultists themselves retreated to the upper areas once it became obvious the city would fall. Now they surround Priestess Halla, a vicious 8th level cleric of Deskari (base off the Death Priest in the NPC codex), who had been making a deal with some of the lower ranking clerics when the armies attacked. Finding herself cut off from the rest of the battle above, she decided to wait below until the paladins were defeated. She has summoned two Incubi to her side and is in the process of sacrificing her cultists so that they might arise more quickly from Abyssal larvae.
Front House: Entering the mines is relatively easy if you know where it is. The house is one of many taken over by cultists and the door to the mines has a bracket where a heavy wooden bar would be slid into place each night to keep the children inside. There is currently no bracket in place.
Cellar: The uppermost room within the mining complex was once a cellar that has been updated with a large table and benches, shelves along one wall containing various tools, a few barrels of food and ale, and a wardrobe full of tattered children's clothing and the sleeping mats they use at night. The clerics themselves slept upstairs. There is no light source in here. The children are expected to live by their darkvision. A long ladder leads down at a ramp-like angle for forty feet over a deep gap to a ledge-like protrusion.
Ledge: This thick six-foot-wide ledge contains down and around the edge of the hole, occasionally broken up and requiring another ramp, with a couple of passageways snaking off of it. The hole is far deeper than this ledge (created by earth shifting magic) reaches. The low intonement of prayers to Deskari can be heard in one of the side passageways.
Chanters Room: Halla believes that Deskari can be appeased by sacrificing one cultist a night for their cowardice. She first carves the group's sins into the sacrifice's skin before bloodily butchering them upon an altar. The sacrifice itself is pleased to be so anointed and chants alongside the rest. There are no children in this room. When Halla sees the PCs, she will attack as will her Incubi and five remaining cultists who have the same stats as those in the nursery.
Bolt Hole: This room is at the end of one of the abandoned passageways. The cultists assumed there was no one here because they figured that the children would have gotten hungry and gone to find them. Instead they all crouch in one small partial tunnel that branches off a long and winding one, using another such room as the latrine. They are hungry and desperate. Mr. Fiprac defends them with his life but if they clearly serve the paladin army or a good God, then he will throw himself at their mercy and hope that they may all be freed. Since the PCs are likely to outright attack him, it's best to give them a Sense Motive DC 15 roll to identify his defensive and protective demeanour, otherwise they may slaughter him before realising their error. There are 35 kids - all tieflings.