[X] Do a broad aerial survey of Horra's lands, take note of his fields, and do an estimate of how many people he should actually be able to support with what he has on hand. When time and discretion allows, attempt to get closer--paying that many fighters with that kind of equipment shouldn't be something that just anyone can afford without being a famously successful leader of warbands--and yet nobody's willing to talk about him, which suggests otherwise. He's clearly not getting that money in an honorable fashion, and people might let some details slip if they don't think they're being watched. Follow the money, see where it points, if you get a chance to track Horra himself though, it's obviously worth a shot--but approaching his home should be the final step in your investigation--who knows what defenses he has on hand.
[x] Conduct a broad aerial survey of Horra's lands, looking for any suspicious earthworks, pits, altars, standing stones, or anything else unusual from the air that might be a ritual site. If that fails, survey the largest longhouse and if Horra leaves, surreptitiously follow him and see what he does.