
SmiloDan RPG Superstar 2012 Top 32 |

I might just combine the banshee with some shadows (probably replacing the Strength Damage with Incorporeality) and giving the shadows little blood drop necklaces and the banshee a blood-red heart-shaped pendant that heals/grants temporary hit points to the banshee every time a shadow damages a PC.
I'm also considering adding a will-o-wisp to the encounter, but the synergy between the banshee's wail and the will-o-wisp's life drain is pretty severe. Especially since the party lacks a primary healer/breath of life caster.
Unless I can come up with a way for the dead PCs to combat death/Morrigan....

Four Horsemen—Death |

There are no quick-and-dirty advancement guidelines I'm aware of. It's pretty much down to adding whatever capabilities you want and recalculating average CR.
That said, it's pretty easy to just plug and chug numbers given the chart without messing with it too much (for instance, don't even mess with hit dice, just give it whatever HP within expected range). As long as you're not stacking a ton of offensive/defensive traits and abilities it should work out okay in general.

Four Horsemen—Death |

Maybe they become disembodied spirits that are able to counteract the shadows in some way. They have a pool of hit points in the form of radiant energy they can either use to grant ablative shields (rather than temp hp which would still be subject to the max hp-drop of life drain effects) to their comrades, or even damage the shadows. So they essentially spend their lingering physical essence to protect their comrades or help damage the remaining foes, fading away more and more as they do. Once the pool is empty, they don't have the strength to actively affect the battle and are gone.
Their souls could be drawn into and held in the aforementioned necklaces and allow for a ritual to return them to their bodies rather than needing full raise dead magic.

SmiloDan RPG Superstar 2012 Top 32 |

** spoiler omitted **