Difficulty is two dials

Every spawn table in the game is indexed by two numbers, challenge and resistance. Havoc ranks just pick which row you get, and rank 30 reaches a row normal Auric never touches.

The mission board shows one difficulty, but the director reads two numbers:

Challenge (C) picks rows in tension, ramp-up, horde composition, monster count, melee aggression and disabler caps.
Resistance (R) picks rows in the specials, horde-timer and patrol-density tables.

A lookup is clamped to the table length, so asking for row 6 of a five-row table returns row 5.

Tables are indexed by challenge or resistancemanagers/difficulty/difficulty_manager.lua:62

DifficultyManager.get_table_entry_by_challenge = function (self, t)
  local challenge = self._challenge

  return t[math.min(#t, challenge)]
end

DifficultyManager.get_challenge = function (self)
  return self._challenge
end

DifficultyManager.get_resistance = function (self)
  return self._resistance

Open managers/difficulty/difficulty_manager.lua at line 62 on GitHub

What each Havoc rank maps to

Havoc rank	Challenge	Resistance	Same as
1-10	3	3	Malice
11-20	4	4	Heresy
21-29	5	4	Damnation
30	5	5	Auric
31-40	5	6	nothing outside Havoc

The bracket code only goes up to R5. Ranks 16-29 add the mutator_increased_difficulty circumstance and ranks 30+ add mutator_highest_difficulty. The second one contains a mutator that adds one more point of resistance. That is why rank 30 plays at R5 and ranks 31-40 play at R6, a column that many specials, horde and patrol tables have but normal play can never reach.

Havoc rank to challenge / resistanceutilities/havoc.lua:118

  if havoc_rank <= 10 then
    difficulty.challenge = 3
    difficulty.resistance = 3
  elseif havoc_rank >= 11 and havoc_rank <= 20 then
    difficulty.challenge = 4
    difficulty.resistance = 4
  elseif havoc_rank >= 21 and havoc_rank <= 30 then
    difficulty.challenge = 5
    difficulty.resistance = 4
  else
    difficulty.challenge = 5
    difficulty.resistance = 5
  end

Open utilities/havoc.lua at line 118 on GitHub

Rank 16+ and 30+ difficulty circumstancesutilities/havoc.lua:89

    increased_difficulty = "mutator_increased_difficulty"
    chosen_circumstances = chosen_circumstances .. ":" .. increased_difficulty
  elseif havoc_rank >= 30 then
    increased_difficulty = "mutator_highest_difficulty"
    chosen_circumstances = chosen_circumstances .. ":" .. increased_difficulty
  end

  local chosen_modifiers = ""
  local current_modifer_level = HavocModifierConfig[havoc_rank]
  local index = 1

Open utilities/havoc.lua at line 89 on GitHub

Rank 30+: +1 resistancesettings/mutator/templates/mutator_modify_pacing_templates.lua:250

  mutator_add_resistance = {
    class = "scripts/managers/mutator/mutators/mutator_modify_pacing",
    init_modify_pacing = {
      modify_resistance = 1,
    },
  },

Open settings/mutator/templates/mutator_modify_pacing_templates.lua at line 250 on GitHub

Rank 30 is a real wall, not a cosmetic step. The specials table jumps from 6 to 8 alive, timers drop from 120-320 to 80-240, the coordinated strike chance doubles to 50%, and quiet phases disappear (see the next chapter). Treat 29 to 30 like moving up a whole difficulty.

On top of these dials, every rank adds a stack of modifiers: tougher elites, more specials alive, faster hordes and so on. The rank calculator shows the exact set for any rank.