A fell cult or deranged conjurer has completed a ritual to summon a demonic creature into the world. Those involved in the summoning lie dead, either a sacrifice or slain by the creature the moment it appeared. The PCs have arrived too late to stop the summoning, but they're still in time to prevent further rampage.
The demon, a mandragora, is a rather weak lesser demon in the grand scheme of things. However, it's likely to be the first demon the PCs have faced in their careers, and it poses a considerable threat to level 3 characters. Play it up as a serious threat, but don't make it out to be some kind of potential major calamity (as a small group of veteran soldiers, at level 5, can fairly easily fell one alone).
The battlefield is the spacious sacrificial chamber used to conjure the demon, likely within a hidden cave or sprawling dungeon. The chamber is mostly dark, though a few candles do illuminate the section containing the dark altar.
Zombies may be featured in the scenario, depending on player count. If they are, these should be the bodies of the conjurer and/or sacrifice(s), rising either as the final echo of the ritual or at the bidding of the demon.
The chamber has a fairly strong echo effect. As such, the PCs can't tell where the Mandragora is located by sound alone, even while it is incanting, and its magic attacks similarly don't reveal its location.
It's up to you what lies outside this chamber to the south. If the PCs try to retreat from the room (while remaining in combat), the Mandragora will simply blast them with its spells from the safety of darkness. If the PCs retreat even further, you could simply have the Mandragora escape a different direction, the PCs having failed (at least initially) to stop it.
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The Mandragora will get out of the lit altar section as fast as possible. It will use its Move and Wink most turns in place of drawing twice in order to stay away from melee PCs (and, with higher player counts, help involve the Zombies). It will almost always avoid illuminated sections, fleeing for the cover of darkness to pelt the PCs with its spells. Though a lesser demon, the Mandragora is clever and will use its magic to the best of its ability. The Altar will prioritize using Bloody Rewards on Scourge rather than Dark Chant due to the significantly higher payoff, though if the Mandragora has an Action in its hand, the bonus from the Altar may wait for that.
The Zombies will move toward and attack whichever PC is closest. The Mandragora is aware of this behavior and will take it into consideration when determining its positioning. The Sacrificial Altar will typically enhace the Zombie's Damage dice with Bloody Rewards since it's a much bigger payoff than adding another d4 to the Hit. But if it's important to lock down the PC, and the PC isn't Vulnerable from the Mandragora's Scourge, then boosting the Hit is acceptable.
The demon is banished, preventing any further mischief and slaughter. The PCs should find some appropriate loot on the conjurer's corpse and/or the trappings of the altar, and gaining Karma would be sensible.
The PCs are forced to flee, suffering an Injury. The mandragora is malevolent and might pursue the PCs, so you could run a chase scene with the potential for further harm (or even death). Alternatively you could handle it narratively. If the battle was closely fought, the mandragora will also escape and seek softer targets for mayhem.
Boost the difficulty by imposing penalties on light sources; at the beginning of every round, all light sources (apart from the candles on the altar) are extinguished, even if magical. This is appropriate for such a fell place, especially if this is an end-of-adventure sort of confrontation.
You can reduce the difficulty by adding more candles in the room, lighting more sections and making others shadowy. This could be appropriate for low player counts where it's harder to get enough illumination to find the slippery demon. It's not too harmful for the Mandragora to be easier to see; it's likely to lose its Incantation, but this results in damage to the canceling PC anyway. Combined with the Altar-boosted Dark Chant (and the strategy of luring PCs to the Altar so it can hit them with Cursed Aura at the beginning of the scenario), this could make the darkness comparatively irrelevant. At high numbers of players, though, this will result in the Mandragora getting picked off quickly.