I've tried this scenario out with my group in the last month, and thought I'd share! It's a variant on the standard skirmish scenario.
At the beginning of each player's recovery phase, roll a die, on 1-5, nothing happens and play continues as normal. On a 6, the Spawn have been discovered! Each player rolls a die: on a 1-2, a single Large Spawn appears. On a 3-4, D3 small spawn appear, and on a 5-6, one Large spawn and one Small spawn appear. These are placed randomly at ground level either by a neutral 3rd party, or by rolling random locations on the board. Spawn have the following stats:
Large Spawn 4 3 0 4 5 3 2 D6 10 4+ Regeneration, Large Target, Fear
Small Spawn 4 2 0 3 4 1 3 D3 10 5+ Regeneration, Fear
Spawn will charge the nearest enemy it can see, and will walk in a random direction if there are no enemies to charge. Spawn will never test for being All Alone, and roll randomly for their attacks during each combat phase. Spawn also treat all Stunned injuries as Knocked Down.
Victory conditions are as normal, but the warbands will gain extra gold from their employers for killing spawn and returning with their heads. Once a spawn is defeated, place a marker with the model that struck the killing blow to represent the severed head. A model will drop the head if attacked, and the marker may be picked up by any model in base contact with it.
The Heads of Large Spawn are worth 6D6 gold, and small spawn are worth 3D6 gold.
XP is awarded as follows:
+1 for surviving the scenario
+1 for being the winning Captain
+1 for any Hero that takes an enemy model/Small Spawn Out of Action
+2 for any Hero or Henchmen Group that takes a Large Spawn Out of Action
Alternatively, a Cult of the Possessed or Carnival of Chaos warband that defeats a small spawn may, instead of killing it, consider it captured, and may add it to their roster as a henchman if they can get the marker off the board. Spawn never gain XP, and keep the same stats as above.
Before the battle, both players have the option of purchasing Torches for their warband members (Heroes or Henchmen) Torches are considered clubs and have Flaming Attacks, which negate the Spawn's Regeneration saves, but will last for the duration of this battle only. Torches cost 2gc each.