Floating before you is a horrid, shriveled human fetus nested within a translucent sphere of dark energy. Its jet-black eyes glitter with intensity as it twitches and spasms slightly, as if in great pain.
Source: Tome of Horror IV Pathfinder
Originally appearing in Rappan Athuk
When a woman who is carrying a child dies in an unholy place her unborn fetus may continue to grow. Infused with dark power and negative energy this fetus will burst through her belly as a new horrid undead creature. Floating within this black sphere of dark energy is an unborn baby who is warped and deprived. The Mordnaissant lives in a constant state of depression and pain. The only relief is through harming others.
The Mordnaissant can be found in any environment but tends to continue to haunt their place of birth. From temples of Dark Gods to the catacombs of the dead. They do avoid places of bright light. Even though not harmful to them they shun it if they can.
Combat: The Mordnaissant is a Tiny Undead with the ability to fly. They are Challenge Rating of 7.
-Death Curse: The individual who finally puts the Mordnaissant to rest must make a DC 20 Will save or be cursed. The cursed character reduces all subsequent experience by 50% until cured.
-Lash of Fury: The Mordnaissant can lash out with negative energy with a ranged touch attack with a 30ft range. The Mordnaissant can pick up to three effects of the lash: whip the flesh(as inflict moderate wounds for 2d8+9 points of damage), whip the mind(1d4 points of intelligence damage), and whip the soul(stunned for 1d4+1 rounds). A DC 22 Fort save is needed to halve damage or resist the effects.
-Lifesense: It notices and locates all living creatures within a 60ft radius
-Pain Wail: As a swift action the Mordnaissant can produce a terrible wailing sound. All creatures within a 20ft radius must make a DC 20 Will save or be dazed for one round. For every 20 points of damage the Mordnaissant takes the DC increases by 1.
-Shield of Agony: The Mordnaissant adds it's Charisma modifier as a deflection bonus to AC.
-Others Abilities: Undead Traits, Channel Resistance +2
The Mordnaissant is a disgusting creature with a gruesome circumstance of birth. The fact that you are fighting an undead fetus is both disturbing and awesome at the same time. They do have an interesting mechanic with the Death Curse ability. Having your experience reduced by 50% is torment to any player. Especially if you are trapped inside a dungeon with no cure readily available. I also like the fact that you have Pain Wail as a swift action and then you can continue to harass your PCs with different Lash of Fury effects each round.
The Mordnaissant can compliment a dungeon with various undead. Maybe filling up a catacomb with skeletons, zombies, and wights with the final confrontation being the undead fetus.
Another possibility is that a Demon Lord or Evil God of Fertility has gathered together woman who are pregnant. Then slaughtered them in a ritualistic fashion to create the Mordnaissant. A den of evil undead babies....how fun....
For the OSR crowd there is a version of the Mordnaissant that is found in the Sword & Wizardry Version of Tome of Horror IV.