Having decided that I a) really liked the very brief time I ran Spears of the Dawn and b) really should restrict the next game I try to run to games I already own the rather than buying yet another OSR game, I shall be fleshing out, in very small ways, a SotD game. One of the other reasons I am planning on using SotD rather than my usual favorite, LotFP w/ B/X and AD&D, is that by picking an OSR game that is heavily flavored away from traditional D&D it is helping me restrain myself from going to far out there with my usual desires for weirdness, that never seem to work out with my groups. Another reason is that while SotD is broadly compatible with all the OSR games, it is much more specifically compatible with the rest of the Sine Nomine Publishing games, Stars Without Number, Other Dust, and most importantly Silent Legions, with all of which I can inject varying amounts of the post-apoc and Sci-Fi that I want in a more general sword and sorcery game. Although I am also tempted to twist everything to be all Metamorphis Alpha too.
My first bit of prep is using outer god generator to flesh out the Gods Below in SotD, which upon my first coming across in 2013, struck me as being ripe for some kind of lovecrafting outer god treatment, so when I saw that SL has an entire chapter devoted to making a new and unique mythos for your game I instantly thought of the Gods Below. I am debating whether I want to pre-gen cults to the Gods Below or Eternal cults, or generate them as needed to fit in with the intended sand-boxery of the campaign.
Thus far I have determined there are three Gods Below who exist imprisoned beneath the world and for reasons all their own granted the Undying Gift the Eternal Lords (the sort of BBEGs of generations past), and grant their cultists access to the Sphere of Blasphemy in SotD, but will also grant dark sorceries and disciplines from SL.
The first of the Under Gods is known to its cultists by such names as "The Seer" and "The Wailing First Arm." It claims dominion over all manner recording speech, which consists primarily blasphemous writings. It is also the primary font from which the eldritch powers granted to cultist flows. Sight and sound, voices and visions, are it's clear forte, but arms can also wield things other than scribes.
The second is known only as "The Bleeding Deviser of the First." It is the Under God of the mind, specifically dreams, nightmares, and visions. Like the other two it is less immediately terrifying than would seem. For while it can inflict horrible visions, it, like the others, is restricted to directly interacting with those foolish enough to truck with them. So, our average person, or hero, should have no worries about terrifying visions of bleeding hellscapes, well not from the Gods Below. It is extremely likely that more than one cultist has been inspired after a blood sacrifice with a ritual or discipline that inflicts nightmares.
The third and final God Below is "The Ashen Tyrant of Day." It is said that the Ashen Tyrant is the responsible for the Night Men that plague the Green Lands of the Lokossa. While sacrifices to the Wailing First Army will be done by threshing, and those to the Bleeding Deviser by exsanguination, sacrifices to the Ashen Tyrant are commonly by immolation, whether by brands or volcano, but can also include the slow suffocation by smoke or by being buried in the ashes of slowly burning fires. It has the greatest direct ability to affect the Upper Lands, but only through the small creatures of the night. It is also said that heat exhaustion, and other sicknesses, are its doing, able to pervert the life giving rays of the Everburning Sun into a horrible death.
Most of the inspiration came from the interplay of the results of the god generation tables and the very brief content on the Gods Below in SotD.
A small thought on using some of the rules from SL in SotD. I am definitely going to use the slaughter die, but only on some magical weapons, and on any more advanced weapons that might make their way into the campaign world. I am still unsure on how exactly I will be integrating SWN and OD. I'm also unsure the extent, if any, I want to include Madness. Since Spears are supposed to hunt down Eternals and cultists, I don't want to to heavily impact the genre assumption of solving a large number of problems with sword and spell. I'd say most likely using the Horror and Occult events, ignoring the Bloodshed events.