Act IV has a hidden mechanic that was never exposed to players. Anyone curious has probably noticed that the roof of the Pandemonium Fortress is always oddly coloured, often with visible flicker and sudden jumps — that is the key clue that leads to this whole mechanism.

Diablo II once had affixes that granted stats based on the time of day. Back then I wanted a way to display the in-game clock, since seeing it would obviously make all this intuitive — and that led to a lot of strange discoveries.

How fast time runs

The internal clock runs from 0 to 359 and is split into four segments:

0 - morning, 1 - dusk, 2 - night, 3 - dawn

Tick 0 starts the morning. Taking it as 6 a.m. and converting to a 24-hour clock gives this:

One tick is four in-game minutes, or five real seconds, and every game always starts at tick 0.

In other words, our poor characters all clock in at six in the morning.

An in-game day is roughly 25 real minutes.

Without the night speed-up it would be 30 real minutes.

An Act III day is 20 real minutes.

Night runs at twice the speed of day.

Act III nights run five times faster than a normal night.

The reason is probably to keep players from spending too long in the dark: beyond the wilderness there are plenty of pitch-black dungeons to explore, and if night also crawled along, the game would feel like one unbroken blackout.

Act IV runs day and night at the same speed, and that speed is 16 times that of other acts.

A full Act IV day-night cycle takes only two minutes.

Time flips by quickly, but Act IV's day-night effect was never actually switched on.

So the world has always felt as if it has neither time nor a day-night cycle.

Lighting colour and brightness

Other acts use this lighting setup:

Colour cycle:

white daylight → purple-magenta dusk → blue night → warm grey-yellow dawn → white daylight

Dusk is purple , night is blue , dawn is warm yellow .

Brightness follows a sine curve:

Brightest at noon, darkest at midnight.

Act V behaves the same way, but its maximum brightness is capped at 170 so the snow doesn't blow out.

Act II's eclipse has a brightness of 32 — darker than midnight.

Act IV's lighting cycle is the most unusual of all:

Act IV sky cycle:

red → yellow → cyan-blue → blue → purple-magenta → warm yellow → red

Sixteen-times speed plus a completely different palette makes Act IV's tone change violently: six eerie colours inside two minutes.

The intent was presumably to create the feeling of a sky racing overhead in the Chaos Sanctuary.

The screenshot shows Act IV at 10:52 in the morning.

Act IV's brightness is hard-coded into the program and does not alternate with day and night — also unusual.

The screenshot shows the darkened City of the Damned.

Act IV segment table:

Compare with other acts:

Act IV's segmentation is idiosyncratic too: dusk and dawn are half as long.

Which also explains the Pandemonium Fortress roof.

[The images compare the fortress lighting with the roof colouring. The two match, and the roof's shifting colours are in fact echoing Act IV's cut lighting]

With Act IV's lighting switched off, a rendering bug made the roof read that hidden lighting anyway. By pure accident you are looking at the cut sky's rapidly shifting colours — and the reason they jump is a handful of bugs.

Bugs:

1. Day-rollover bug (all acts): dawn ends up only half as long as designed, jumping from 04:30 straight to 06:00 — in Act IV from 05:20 to 06:00 — and the lighting snaps with it. This is the flash just before sunrise when the screen suddenly brightens; someone has probably noticed it while playing.
2. Time-rewind bug (Act IV only): the Act IV tick timer frequently runs backwards, so the sky colours rewind as well.

- near 180 it rewinds to 160

- near 190 it rewinds to 180

- near 340 it rewinds to 320

3. Segment-transition bug (Act IV only): the smooth colour interpolation between segments never happens, so the colours jump harshly.

I have fixed these as well; the colour transitions look comfortable now.

Summary

1. Act IV runs time at 16x, and its segmentation is unusual too: dusk and dawn are 50% shorter.
2. Act IV's brightness is locked for the whole day and varies only by area; day and night never alternate in brightness.
3. Act IV's day-night cycle appears as rapid, extreme colour shifts; the map's brightness sets the colour intensity, and the Pandemonium Fortress is the most intense.
4. The Outer Steppes' brightness equals other acts' night; the later maps are darker than the darkest night elsewhere, and the underground section is twice as dark as Act II's eclipse — Act IV is the darkest act in the game.

Point 1 exists in the shipping game but is imperceptible: retail never enabled the later features. The lighting colour never changes and is not colourful, brightness is fixed as well, and none of it is anywhere near as dark as the cut version.

These traits were clearly designed on purpose, but enabling them produced too many bugs, so the mechanic was abandoned halfway.

I did not use the remaster to demonstrate this, because it does not seem to have recreated or fixed this feature.

I fixed the time and colour-jump bugs so the mechanic can display properly in my mod. Act IV will be the most distinctive chapter of The Reliquary.

Next I will write about my Act IV remake plan. Stay tuned.