Ever wondered why there is literally a sun damage enchantment in this game, but the sun itself does no sun damage (without Auri-El's bow) when in Oblivion and Morrowind it did.
You might be a smart-arse and say it is a different strain of vampirism but Skyrim's strain (lore-wise at least) is still burnt by the sun. Why do you think Harkon was so hell-bent on blotting it out? A slight debuff hardly qualifies as the "Tyranny of the Sun".

What Does this Mod Do?

Vampires should now fear the light of Magnus. The sun does constant damage to NPC vampires. Pureblood vampires take less damage per second (DPS). Low (~0.5-2 DPS), Medium (~2-6 DPS) and High (~4-18 DPS) options available. I recommend the low option, but it's up to you.

All non-Volkihar Clan NPC vampires under level 45 will flee from combat while sustaining sun damage. The Sun Damage effect will stop when the vampire has 10% health or less but the fear effect will remain.

Serana will no longer take sun damage if you cure her. Optional file to make all undead take sun damage (but no fear because death hounds, draugr and skeletons etc. are non-sapient)

These effects are applied between 5 am and 7 pm if the sun is out (i.e. not heavy rain, snow or cloud cover).

These effects DO NOT apply to the player for compatibility reasons. I suggest you use Better Vampires if you want this. Optional Patch to extend the same sun burning visuals when you take sun damage from Better Vampires and makes sure NPCs you converted to vampires are also burned by the sun.

All vampires will now also turn to ash when killed.

Vampires that spend time in broad daylight in vanilla (namely Hern and Hert from Halfmoon Mill) have their AI packages altered to make them sleep during the day and work during the night.

Immersive Citizens also alters their packages to do this but not quite so that they are away from sunlight all the time. Optional patch for this.

Missives add a radiant "hunt vampire" quest. These vampires travel from an inn to a vampire lair. Optional patch to make it so that they stay indoors during the day.

I've also added and hand placed three relevant lore books with unique custom covers. "The Sky is Not Blue" is a Restoration skill book, "Exegesis of Merid-Nunda" is an Alteration skill book and "The Rainbow-Princess Fights the World-Eater" is a bedtime story for ancient Nord children (and the ending changes slightly depending on your character).
"The Sky is Not Blue" is found in Temples and Shrines to Kynareth (but sometimes hidden) and some Vampire Lairs.
"Exegesis of Merid-Nunda" is found in the ruins of Meridia's Temple in Kilkreath and Falion's House.
"The Rainbow-Princess Fights the World-Eater" is found in ancient Nord city ruins.
"Exegesis of Merid-Nunda" is from ESO and can be read here, but the content of "The Sky is Not Blue" and "The Rainbow-Princess Fights the World-Eater" have been taken and modified slightly from here and here, respectively.

This mod has no constantly running scripts and all the effects are applied using conditions so should be very performance-friendly.

Installation/Uninstallation

Install one main file and whichever patch files you need. Use a mod manager or extract it into the data folder. Can be installed at any time.

No constantly running scripts so can be uninstalled any time too. Probably best to uninstall in an interior during the night time while not in combat with vampires. Use a mod manager or delete the esp and all the relevant files.

Compatibility

This mod works by altering vampire race records. As a result, IT WILL CONFLICT WITH ANY RACE OVERHAUL or mod that alters the race records. Optional patch for Disparity. Might make other race overhaul patches if requested.

As mentioned earlier patches available for:
Better Vampires to make player burning visuals the same as NPCs.
Immersive Citizens to make sure Hern and Hert don't go out during the day.
Disparity to make sure the mods changes to the race records are kept.
Missives to make sure radiant vampires don't go out during the day.
Also, a patch for Lawbringer's patch for missives as they conflict because of the radiant exclusions. Only use my one.

I have used the turn to ash feature from Better Vampire NPCS but this mod is still 100% compatible with it. If you are asked to overwrite scripts when installing it doesn't matter if you do or don't as the script is identical.

Lots of .esps if you need many patches. Nothing I can do about this, but each optional esp is very small and will probably get integrated into a bashed patch or you can merge them yourself.

Best to have high up in your load order for compatibility. All changes from USLEEP are carried forwards.

If you have many mods please look at this one with TesVedit to check if there are conflicting records.

Let me know if you discover an incompatibility I'm unaware of and I might patch it.

Known Issues

I don't know how to make all vampires run away from the sun when taking damage (like Run For Your Lives or Wet and Cold). it would require many constantly running scripts, a lot of effort on my part to figure it out and be a compatibility nightmare. As a result, vampires that are taking sun damage will continue acting as normal outside of combat. I made the effects stop if the vampire has 10% or less health due to this for compatibility. This can sometimes be bypassed however if you wait 1 hour near a vampire that is being idle and has low (but >10% health). In this case, the vampire will be killed by the sun. The package changes I have done prevent certain vanilla vampires from strolling around in the daytime as well for this reason.

Dawnguard has a radiant masquerading vampire quest, "Hide And Seek". It is set up very weirdly; I tried to alter the radiant vampire's AI packages and the quest but I couldn't. As a result, they will sometimes chill outside and burn from sun damage acting like nothing is happening. They are exceptionally stupid in dialogue so this could probably be used as an in-game explanation.

If you console commands or the add item menu to get the book "The Rainbow-Princess Fights the World-Eater" it will cut off at the end. You must find it to read the ending.

Please let me know if you find anything else.

Credits

Thanks to Brehanin for letting me borrow the script that makes vampires turn to dust on death. Check out and endorse his Better Vampire mods - they are great!
Thanks to DanielCoffey for Book Covers Skyrim - Lost Library which I used as a base in creating my book covers.
Thanks to u/Sevatar___ and an unknown user for the stories in the lore books.