Some edits I don't remember making. Just uploading it to not invalidate my saves. Below is the original description by rottensunlight@gmail.com.

 This addon is supposed to alter the cursed/Darkness tree

to make it useful, while changing as little as possible. I have long
since gone past "as little as possible". I also added the Inner Darkness
tree, an advanced tree meant to supplement Darkness the way that Advanced
Shadowmancy supports the Shadows tree.

-- Creeping Darkness: I lowered the cooldown, I set the number of seeds

            of darkness to increase appropriately with talent
            level. The darkness now inherits a weaker version
            of the debuffs of other debuffing abilities you
            learn once those talents reach level 4. In
            particular, Dark Torrent, and Dark Tendrils. The
            strength of the debuffs are determined by the
            talent level of the talents from which the
            debuffs are stolen, and your mindpower. The
            density of the darkness is based on mindpower and
            talent level, filling up to 75% of the given aoe 
            at 100 mind power and talent level 5.

-- Dark Vision: I increased the max range of the vision to 7

            squares, to enable one to target Agony and
            Hateful Whisper correctly. I also boosted the
            movement speed buff slightly.

-- Dark Torrent: I set the ability to always leave creep behind

            instead of randomizing a core part of the ability.
            I also increased the damage very slightly.

-- Dark Tendrils: I made it faster, with scaling based on talent

            level, and the damage of the darkness it leaves
            behind is improved. I also added an extra
            tendril at talent level 6.

-- Grudge: This gives you a hate/life heal, while

            inflicting the Grudge debuff, which will take
            back the life it healed, plus a little more, 
            making you bleed creeping darkness all the while.

-- Veil Walk: This is just a short range teleport, with the

            ability to switch your place with opponents. It
            gains range with talent level. It is instant if
            you are stepping into your creeping dark. It
            also cuts fear immunity, dark resist and mind
            save when you switch with an enemy.

-- Heartless: This sustained ability burns hate to deal a

            small amount of dark damage on top of every
            blow you deal, and also makes enemies leave
            behind pools of darkness when you kill them.

-- Dark Blood: This ability is pure damage smearing. It collects

            all the damage you take in a single pool and
            portions it out to you over the following turns
            at a set rate.

I have also unlocked the Fears tree, and changed it to be like the version
of Fears in HousePet's afflicted rework. However, the fear effects
themselves have been altered. In particular, the fears that inflict
damage are now allowed to crit, and some of the fears were locked so as to
not inflict brainlock. This has been removed.

I have significantly buffed Gestures to the point of being absolutely brokt,
in the hopes that they'll make a possible alternative to psiblades. I'll
just have to keep buffing them until you don't have to take psiblades, and
then tune them until they aren't crushingly oppressive.

Adds a new evolution for Shadowblades, the Umbral Incantor.

As an Umbral Incantor, the Shadowblade will regenerate 1 mana per turn and gain several bonuses to their talents if they are equipped with a staff:

- Shadow Combat: the damage will also be triggered when you cast Channel Staff, and when triggered this way can be a spell critical
- Shadowstep: teleport back to starting location after the attack
- lluminate: will use Shadow Dance without cost or triggering cooldown
- Invisibility: will be cast instantly without cost when Stealth is deactivated (still triggers cooldown)
- Shadow Grasp: will pin for 3 turns instead of pulling
- Shadow Veil: rather than blink and attack, you will cast Shadow Grasp on targets selected (without cost and ignoring cooldown and failure chances)
- Lethality: staffs gain a Dexterity damage modifier equal to twice the crit bonus
- Expose Weakness: (if not on cooldown) will be used instantly when casting Shadowstep (before the return teleport), using stamina as usual and going on cooldown for twice the usual duration
- Blade Flurry: (while sustained) also gain spell cooldown reduction equal to the attack speed bonus and an equal chance on spell hit to cast Channel Staff
- Snap: also affect an equal number of spells ("spell/" category talents)

Adds a new undead race:

Weight: 134953
Wip add-on, everything works as of now but I want to re-write some of the code and rebalance it. Always open for feedback!
Meet the gloomblades, a tainted race of undead with an affinity for darkness. With stat boosts to str and cunning.
My first add-on, so Rip it apart! Let me know what sucks and what could be better! :)

Credits

Wip-
Dr root,
Min,
Nsrr,
Deathknight class addon,
Qlass pack
Dark god

And all the other wonderful people on the Tome4 Discord!

As in vanilla, all talent icons are from http://game-icons.net/ and licensed under CC-BY 3.0. Class icons are from https://opengameart.org/content/painterly-spell-icons-part-4 and https://opengameart.org/content/11-planet-sci-fi-set .

Sounds:
http://freesound.org/people/pingel/sounds/2689/ CC 0
http://freesound.org/people/artifact/sounds/29675/ CC BY 3.0
http://freesound.org/people/BOBtheROSS/sounds/244358/ CC BY 3.0
http://freesound.org/people/CGEffex/sounds/98335/ CC-BY 3.0
http://freesound.org/people/Ephemeral_Rift/sounds/77370/ CC BY-NC 3.0
http://freesound.org/people/jameswrowles/sounds/248219/ CC BY 3.0
http://freesound.org/people/Julien%20Matthey/sounds/105016/ CC 0
http://freesound.org/people/misosound/sounds/251759/ CC 0
http://freesound.org/people/nicStage/sounds/64460/ CC BY 3.0
http://freesound.org/people/benjaminharveydesign/sounds/315914/ CC 0
http://freesound.org/people/misosound/sounds/251759/ CC 0
http://freesound.org/people/werra/sounds/232130/ CC 0
http://freesound.org/people/bspiller5/sounds/157616/ CC 0
http://freesound.org/people/qubodup/sounds/219566/ CC-BY 3.0
http://freesound.org/people/gezortenplotz/sounds/20135/ CC-BY 3.0

Changelog

1.0.0 - Release

NOTE: As of ToME version 1.7.3, this fix has been incorporated into the base game, and so should be deactivated/uninstalled.

Tweaks LOS code for both NPCs and the player to properly and consistently respect Dark Vision when determining whether Creeping Darkness blocks sight.

Targeting and projection now properly treat Creeping Darkness as transparent if you have Dark Vision, just like any other tile that can be seen through, eliminating awkward behavior when targeting diagonally and/or around corners. If you do not have Dark Vision, these tiles will be treated as opaque. Additionally, NPCs will properly be able to see and target through their own creeping darkness without hindrance. Similar effects to Creeping Darkness such as Cursed Miasma should also be corrected by this addon.

This addon hard-overwrites engine and module functions. As such, I cannot fully guarantee compatibility or stability.

This addon is meant to make demon seed and demon blade deal at least
partially darkness damage, to improve synergy with Doom Covenant

Adds the Dark Priest, a mage subclass.

Even in an age of supposed prosperity, there are many who lose their lives in vain. Those who tend the wounded, returned home from battle or adventure, are highly skilled in the arts of healing, many using arcane techniques. But there are some who have seen too much. They have formed a brotherhood of those who have accepted the Dark Truth. Eyal has but one true God, and He demands sacrifices. The Dark Priest and his brethren tap into arcane energies to deliver the bad word unto all the people of Maj'Eyal. They manipulate darkness and life energy to convey their message of doom and despair. When that doesn't work, carrying a big stick helps, too.

Dark Priests use Magic and Cunning as their primary stats. They focus on Darkness and Arcane spells, and are proficient in staff combat, as well. They also have command of a unique kind of magic, Unhealing. Unhealing is negative healing and such functions differently than damage. It will not be affected by any type of damage mitigation and will be increased by the target's healing modifier.
(full descriptions in the forum: http://forums.te4.org/viewtopic.php?f=50&t=45460 )

Adds a new Afflicted Subclass proficient in the Cursed Talent Trees Darkness, Force of Will, Gloom, Cursed Form and Dark Sustenance and the Corruption Talent Trees Vim, Doom Covenant, Black Magic and Curses. They also possess the Cunning Talent Trees Stealth and Survival. This Addon requires ASHES OF URH'ROK to be enabled in the Addons Menu. THIS ADDON DOESN'T GRANT YOU ANY FILE FROM ASHES OF URH'ROK! It's only a Subclass.