code:

/**
 * Choose a random, spooky hell effect message, print it, and make a loud noise
 * if appropriate. (1/6 chance of loud noise.)
 */
static void _hell_effect_noise()
{
    const bool loud = one_chance_in(6) && !silenced(you.pos());
    string msg = getMiscString(loud ? "hell_effect_noisy"
                                    : "hell_effect_quiet");
    if (msg.empty())
        msg = "Something hellishly buggy happens.";

    mprf(MSGCH_HELL_EFFECT, "%s", msg.c_str());
    if (loud)
        noisy(15, you.pos());
}

/**
 * Choose a random miscast effect (from a weighted list) & apply it to the
 * player.
 */
static void _random_hell_miscast()
{
    const spschool which_miscast
        = random_choose_weighted(8, spschool::necromancy,
                                 4, spschool::summoning,
                                 2, spschool::conjuration,
                                 1, spschool::charms,
                                 1, spschool::hexes);

    const int pow = 4 + random2(6);
    const int fail = random2avg(97, 3);
    MiscastEffect(&you, nullptr, {miscast_source::hell_effect}, which_miscast,
                  pow, fail,
                  "the effects of Hell");
}

/// The thematically appropriate hell effects for a given hell branch.
struct hell_effect_spec
{
    /// The type of greater demon to spawn from hell effects.
    vector<monster_type> fiend_types;
    /// The appropriate theme of miscast effects to toss at the player.
    spschool miscast_type;
    /// A weighted list of lesser creatures to spawn.
    vector<pair<monster_type, int>> minor_summons;
};

/// Hell effects for each branch of hell
static map<branch_type, hell_effect_spec> hell_effects_by_branch =
{
    { BRANCH_DIS, { {RANDOM_DEMON_GREATER}, spschool::earth, {
        { RANDOM_MONSTER, 100 }, // TODO
    }}},
    { BRANCH_GEHENNA, { {MONS_BRIMSTONE_FIEND}, spschool::fire, {
        { RANDOM_MONSTER, 100 }, // TODO
    }}},
    { BRANCH_COCYTUS, { {MONS_ICE_FIEND, MONS_SHARD_SHRIKE}, spschool::ice, {
        // total weight 100
        { MONS_ZOMBIE, 15 },
        { MONS_SKELETON, 10 },
        { MONS_SIMULACRUM, 10 },
        { MONS_FREEZING_WRAITH, 10 },
        { MONS_FLYING_SKULL, 10 },
        { MONS_TORMENTOR, 10 },
        { MONS_REAPER, 10 },
        { MONS_BONE_DRAGON, 5 },
        { MONS_ICE_DRAGON, 5 },
        { MONS_BLIZZARD_DEMON, 5 },
        { MONS_ICE_DEVIL, 5 },
    }}},
    { BRANCH_TARTARUS, { {MONS_TZITZIMITL}, spschool::necromancy, {
        { RANDOM_MONSTER, 100 }, // TODO
    }}},
};

/**
 * Either dump a fiend or a hell-appropriate miscast effect on the player.
 *
 * 40% chance of fiend, 60% chance of miscast.
 */
static void _themed_hell_summon_or_miscast()
{
    const hell_effect_spec *spec = map_find(hell_effects_by_branch,
                                            you.where_are_you);
    if (!spec)
        die("Attempting to call down a hell effect in a non-hellish branch.");

    if (x_chance_in_y(2, 5))
    {
        const monster_type fiend
            = spec->fiend_types[random2(spec->fiend_types.size())];
        create_monster(
                       mgen_data::hostile_at(fiend, true, you.pos())
                       .set_non_actor_summoner("the effects of Hell"));
    }
    else
    {
        const int pow = 4 + random2(6);
        const int fail = random2avg(97, 3);
        MiscastEffect(&you, nullptr, {miscast_source::hell_effect},
                      spec->miscast_type, pow, fail,
                      "the effects of Hell");
    }
}

/**
 * Try to summon at some number of random spawns from the current branch, to
 * harass the player & give them easy xp/TSO piety. Occasionally, to kill them.
 *
 * Min zero, max five, average 1.67.
 *
 * Can and does summon bands as individual spawns.
 */
static void _minor_hell_summons()
{
    hell_effect_spec *spec = map_find(hell_effects_by_branch,
                                      you.where_are_you);
    if (!spec)
        die("Attempting to call down a hell effect in a non-hellish branch.");

    // Try to summon at least one and up to five random monsters. {dlb}
    mgen_data mg;
    mg.pos = you.pos();
    mg.foe = MHITYOU;
    mg.non_actor_summoner = "the effects of Hell";
    create_monster(mg);

    for (int i = 0; i < 4; ++i)
    {
        if (one_chance_in(3))
        {
            monster_type *type
                = random_choose_weighted(spec->minor_summons);
            ASSERT(type);
            mg.cls = *type;
            create_monster(mg);
        }
    }
}

/// Nasty things happen to people who spend too long in Hell.
static void _hell_effects(int /*time_delta*/)
{
    if (!player_in_hell())
        return;

    // 50% chance at max piety
    if (have_passive(passive_t::resist_hell_effects)
        && x_chance_in_y(you.piety, MAX_PIETY * 2) || is_sanctuary(you.pos()))
    {
        simple_god_message("'s power protects you from the chaos of Hell!");
        return;
    }

    _hell_effect_noise();

    if (one_chance_in(3))
        _random_hell_miscast();
    else if (x_chance_in_y(5, 9))
        _themed_hell_summon_or_miscast();

    if (one_chance_in(3))   // NB: No "else"
        _minor_hell_summons();
}