This sounds so cool! I’m running a nature cult game IRL with radiant and sinister as my deity’s precepts. As an example, we have a tycherosi whisperer whose a member of the Foundation with Strange Methods. His thesis project was a magic ring carved out of Brightstone’s, uh, brightstone that could morph into weapons.
So you mentioned the ritual for the wild hunt-maybe that could be some sort of supernatural cohort with their own tags (are they the same tags as the Horned Lord? Are the features for the Horned Lord drawbacks for the wild hunt? A neat way to flavour them could be that the cohort that’s summoned by the ritual varies in scale, potency, force etc., with more stress pertaining to a better version of the wild hunt summoned (all according the magnitude stuff on p. 222 I think?). As for drawbacks, cohorts come with their own drawbacks, but if you dont think thats enough there could always be some fun clocks like “The Hunt comes for those that Call it 1/8” (well, maybe something a bit shorter) or maybe its as simple as 1/4 accosted by the Spirit Wardens like with electroplasm rounds. At the same time, consider the implications in the fiction of summoning something like the wild hunt in the Dusk. Is it something that would make waves in the supernatural community? Asking your players questions could help with come up with some consequences. For example, a simple question that could really be inspirational for questions: when called, does the hunt recognize/exclude people from its hunt or is it indiscriminate? These could help lead you to some natural consequences that could be taken care of with clocks, heat fortune rolls etc.
A really fun drawback could be some sort of clock that sees the Wild hunt chasing after the person who summoned them, represented by some sort of clock set at your discretion. Going back to the idea that the amount of stress informs the quality/tier of the cohort called, you could make a fortune roll using the tier/quality of the Wild Hunt called as the number of dice that advance the clock during downtime. it could be a really fun, engaging way of having the player’s actions come back to literally haunt them.
Now, as for the spear, it sounds like a really high quality weapon. Do you envision it as something that empowers the wielder? It would obviously be a fine weapon, though you might want to set it at a higher quality as its the weapon of deity/object of worship (Tier +2 maybe?). Maybe the quality and/or force increases the more pieces of the Horned Lord raiment the cult tracks down (you mentioned they have a helm, sounds like a good start, especially with how you mentioned the horn as well). In terms of drawbacks, you said the Horned Lord was a hunter of demons. What better prey for the deity that hunts demons than the biggest demons of them all: the Leviathans. Maybe it has to be consecrated in the blood of the oldest, strongest demons that are about. Maybe it has to kill certain kinds of demons that pertain to certain kinds of elements (maybe the spear hunts whatever kind of demon is the Horned Lord’s opposite? might be a bit narrow).
Tradtionally, horns started and ended the hunt. To that end, maybe the dirge from the horn gives the crew some kind of set up action on certain plans for scores, letting take advantage of the distractions caused by it to sneak past guards, to follow up on an enemies burst ears with an assault or maybe even makes the voices of the cult seem more reasonable as their words take root at the outset of a negotiation. The drawback in this case would be obvious: the horn’s call is indiscriminate, anyone taking advantage of it also suffers from the same drawback it inflicts at the beginning of the score (resist-able, of course ).
Anyway those are just some ideas, sorry if the post is incoherent for how long it is. The Cult crew is easily my favourite one, its always great to see what other people get up to playing one!