KILLING AND AVOIDING BEING KILLED
Combat works basically however you can remember it from OD&D, as best as you can remember. You won't be able to look up a rule so you aren't tempted. The following additional rules apply:
Environment as Weapon: Pushing a blind Goblin into a spike of exposed rebar or a stagnant pool of caustic water is an instant save versus death.
Weapon Damage: Most weapons deal 1d6 damage. A smuggled shank, a heavy rock, a rusted pipe, file-sharpened teeth. The weapon is less important than the desperation of the wielder. On a natural 20 with an edged weapon, deal a bleeding wound that causes 1 extra point of damage until bandaged. If the weapon is made of bone, it shatters on a natural 1.
Blood: If a bleeding wound is left untreated, the smell draws the things that live deeper below. Encounters appear on 2-in-6 instead of 1-in-6.
Tetanus: Iron found down here is pitted, flaking, and orange. Striking an unarmored foe with a rusted blade forces a Save vs Poison. Failure results in lockjaw and spasming limbs 1d6 turns later, rendering the victim unable to speak, cast, or fight until cured.
Darkness: Striking a torch, lantern, or glowing locus from an enemy's hand requires no roll, only a declared intent and a forgo of damage. Darkness swallows the room instantly. Anyone caught fighting in total darkness rolls all attacks and Morale saves at disadvantage.
Claustrophobia: If you are fighting in a tunnel too narrow to swing your arms, you can only use thrusting weapons (shanks, sharpened femur bones). Anyone attempting to swing a blunt object attacks with a penalty.
Panic and Stampede: In tight corridors, a routing enemy or fleeing ally knocks over anyone in their path who fails a Save vs Paralysis. Trampled creatures take 1d6 damage and are moved to the bottom of the initiative order next round.
The Drop: Attacking from above (dropping from a ventilation shaft or a rusted grate) grants an immediate, free attack before initiative is even rolled.
More important than any combat rule is the environment. Foes would never attack you in a well-lit, large square room empty of hazards. They will stalk you until you are in near total darkness, tiptoeing along a narrow ledge, or crossing a bridge over putrid water. Drowning a dwarf in scrap-plate armor is much easier than fighting it in open combat. Fights aren't honorable. Nothing much is.