[Ah, Gregorius can avoid the apoplexy, then.]
Demeter's key portfolio is grain and bread, along with their inverse, starvation and hunger (see: nearly wiping out humanity by making crops no longer grow in her grief at her loss of Persephone).
There's not huge amounts of scope to make that useful in the current situation, but it might be possible to get fields of grain in the area to report when they were being ridden through. This will be more useful at some times of the year than others, but is at least better than nothing.
[OOC: I am assuming that Demeter as a very powerful faerie broadly has Focus powers that enable her to use Faerie Powers as per RoP:F (pg 124 onwards) and the Sympathy traits listed for her suggested descendants in The Sundered Eagle pg 127 - positive traits Fertility, Agriculture, Domesticated Animals and negative trait Winter. Plus a few other Demeter specific powers.]
As for the deal, Demeter would be interested in the following:
- An increase in the yield of the local harvest (particularly the barley harvest), and this being associated with her by the locals
- The popularisation of the farming of her sacred animal, the pig
- Rejuvenation of her mysteries
- Rebuilding of the temple to Neptune in the covenant as a temple to her
That isn't an exhaustive list, which is good as all of them have obvious problems - there is a fundamental issue that Demeter is a pagan goddess, and Halia a priestess, so her worship is a big part of what she's interested in. It might be possible to come up with something more acceptable / less likely to cause problems, but Viola is going to have to get creative.
Keyx is able to be bargained down a bit from his initial ridiculousness, but Viola does get the impression that even if he's grudgingly grateful to her for helping his Mum, he still doesn't like or trust her that much. He and his brothers can send word if a band of horses crosses any of their creeks, but they're not going to muck around trying to make them impassable. In exchange, they want a barrel of beer delivered to a location of their choosing once a month, and if they do warn of attackers and the covenant survives, a big party lasting at least three days and a share of the loot.