I'm not so sure "Abyss" qualifies as a favored terrain. There is a gazetteer of the Abyss in the 4th book and it covers some extremely varied terrain types for various realms within the Abyss, and I happen to know that even within a single realm there can be vast stretches of land that change from one type of terrain to another very suddenly.
If you could somehow tweak it so you can get favored terrain that qualifies across the Abyss and have your Terrain Dominance affect that, then sure I guess it would be pretty powerful.
I'm not so sure "Abyss" qualifies as a favored terrain. There is a gazetteer of the Abyss in the 4th book and it covers some extremely varied terrain types for various realms within the Abyss, and I happen to know that even within a single realm there can be vast stretches of land that change from one type of terrain to another very suddenly.
If you could somehow tweak it so you can get favored terrain that qualifies across the Abyss and have your Terrain Dominance affect that, then sure I guess it would be pretty powerful.
The Abyss qualifies as favored terrain Plane (pick one, other than Material Plane). Terrains can cover multiple terrains but you only get you highest bonus. So you could by a forest in the Abyss. Forest would count as favored terrain as would abyss an you take the higher of the two. You same situation with a forest in winter. In winter it counts a snow so Cold Terrian applies as does forest.
The horizon walk in this AP would be extremely powerful against demons. As GM I'd talk to the play have them limit the bonus going into Abyss to something reasonable like the bonus can only be 2 higher than all you other favored terrains.
The Horizon Walker still does its Nightcrawler thing. Better, in fact, with the Dimensional Assault line of feats Pathfinder created.