For a full example of a slayer/horizon walker build:
Half Orc Slayer
Sacred Tattoo, Shaman's Apprentice (endurance)
1.) Slayer 1: Power Attack
2.) Slayer 2: Slayer Talent: TWF
3.) Slayer 3: Two-weapon Rend, Sneak Attack: 1d6
4.) Slayer 4: Slayer Talent: Trapfinding
5.) Slayer 5: Double Slice
6.) Slayer 6: Slayer Talent: Improved TWF, Sneak attack: 2d6
7.) Horizon Walker 1, Favored Terrain: Urban, Feat: Outflank
8.) Horizon Walker 2, Favored Terrain: Astral, Terrain Mastery: Astral
9.) Horizon Walker 3, Terrain Dominance: Astral, Feat: Dimensional Agility
10.) Slayer 7
11.) Slayer 8: Combat Trick (Improved Critical), Feat: Dimensional Assault
12: Slayer 9: Sneak Attack 3d6
13.) Slayer 10:Slayer Talent: Greater TWF, Feat: Dimensional Dervish
14.) Slayer 11:
15.) Slayer 12: Sneak Attack 4d6, Feat: Dimensional Savant
I think that's about as good as you can do. I spent a few talents on general utility (trapfinding, evasion, urban/astral hide in plain sight), but the rest of it's just straight damage, homie.
A few nice twists:
Half-orcs get a 1d8/1d8 double weapon, so no particular need to spend a feat on sawtooth sabre.
You can get endurance as a racial trait, so no need to blow a feat on it.
Strength-dependence means that Two-Weapon Rend is basically another attack.
Power attack works dandy.
Plenty of free feats and talents in the build. I choose trapfinding, evasion, slayer camouflage and two-weapon rend/double slice, but you can do whatever.
You _will_ be glass cannony, but your nova potential is absolutely ridiculous. This build might be even better with ranger as a base, just for all the favored enemies you could have