A delver is focused on overcoming the obstacles of a dungeon and using the cramped quarters to greater effect.
Combat Focus I
At 3rd level, you gain Trap Sense (as the rogue class feature) with a +1 bonus. This bonus increases by +1 each time you improve your Combat Focus.
You also add twice you Combat Focus level to Strength checks to break a door, wall, or similar obstacle (+2 at 3rd level, +4 at 5th, and so forth).
In addition, you can use the narrow tunnels of a dungeon to your advantage in combat, battering your enemies against the walls. Whenever you bull rush or otherwise force an opponent to move into a wall or other solid object, the momentum of the impact deals bludgeoning damage to the opponent equal to 1d6 + half your Strength bonus. If you push the opponent into another creature, both creatures take half this damage and you may continue the bull rush as normal.
Combat Focus II
At 5th level, you can use the narrow tunnels of a dungeon to greater advantage. The damage you deal with your bull rush increases to 2d6 + your full Strength bonus.
Combat Focus III
At 9th level, you learn to use obstacles to control the area around you. You gain a +1 dodge bonus to AC as long as you are adjacent to a wall.
You also increase the damage for crushing an opponent into a wall to 4d6.
Combat Focus IV
When you reach 13th level, the damage for crushing an opponent into a wall increases to 6d6.
Combat Focus V
When you reach 17th level, your dodge bonus while adjacent to a wall increases to +2, and the damage for crushing an opponent into a wall increases to 10d6.