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).
Combat Focus II
At 5th level, you can use the narrow tunnels of a dungeon to greater advantage. Whenever you bull rush or otherwise force an opponent to move into a wall or other solid object, the momentum of the impact deals 2d6 bludgeoning damage to the opponent. If you push the opponent into another creature, both creatures take half this damage.
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 (2d6 each if knocking them into another creature).
Combat Focus IV
When you reach 13th level, the damage for crushing an opponent into a wall increases to 6d6 (3d6 each if knocking them into another creature).
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 (5d6 each if knocking them into another creature).