Lets say I got a Ranger 10 / Horizon Walker 10.
Favorite Terrain
Ranger Lv 3 - Underground (Caves / Dungeons)
Ranger Lv 8 - Forest
Horizon Walker Lv 1 - Mountain
Horizon Walker Lv 2 - Favorite Trerrain
Horizon Walker Lv 3 Terrain Dominance - Underground
Horizon Walker Lv 4 - Favorite Trerrain
Horizon Walker Lv 5 - Favorite Trerrain
Horizon Walker Lv 6 Terrain Dominance - Forest
Horizon Walker Lv 7 - Favorite Trerrain
Horizon Walker Lv 8 - Favorite Trerrain
Horizon Walker Lv 9 Terrain Dominance - Mountain
Horizon Walker Lv 10 - Favorite Trerrain
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Terrain Bonus
+2 bonus on initiative checks and Knowledge (geography), Perception, Stealth, and Survival skill
Question:
At level 20 Since he got 7 Terrains by Horizon Walker and 2 by Ranger would means that in the Underground Terrain would have +18 bonus on initiative checks and Knowledge (geography), Perception, Stealth, and Survival skill and in Forest +16 bonus on initiative checks and Knowledge (geography), Perception, Stealth, and Survival skill and so on
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Favorite Enemy
+2 bonus on Bluff, Knowledge, Perception, Sense Motive, and Survival checks against creatures of his selected type. Likewise, he gets a +2 bonus on weapon attack and damage rolls against them.
Terrain Dominance
At 3rd level, a horizon walker learns total dominance over one terrain he has already selected for terrain mastery. When dealing with creatures native to that terrain, the horizon walker treats his favored terrain bonus for that terrain as a favored enemy bonus (as the ranger class feature) against those creatures. This bonus overlaps (does not stack with) bonuses gained when fighting a favored enemy.
If Underground is chosen as Terrain Dominance and it overlaps with Favorite enemy, what this suppose to mean?
1. Only Overlaps Perception and Survival since those two skills concur and since he got 9 Terrains the character would end with +18 Perception and Survival for the chosen Favorite enemies?
2. What would happend with all other bonus? (+2 bonus on Bluff, Knowledge, Sense Motive, checks against creatures of his selected type. Likewise, he gets a +2 bonus on weapon attack and damage rolls against them.) Would these bonus also be affected by the Dominance Terrain?
So with the Underground Terrain (9 Terrains total) chosen at Level 3 Ranger would have +18 bonus on Bluff, Knowledge, Sense Motive, checks against creatures of his selected type. Likewise, he gets a +18 bonus on weapon attack and damage rolls against them.
sorry for keeping it so short:
bonuses should stack in general, except when noted different, so you're right, overlap adds.
If you want to focus on favorite terrain this much, why not take a rogue?
they got favorite terrain as trick and thus can gain a lot more than ranger, in fact you'll run out of terrains to pick if you take it whenever you can.
There is a thread about it, the bonus is very high, but not very gamebreaking.
have fun being a 1.5 trick pony.