Foreword

Time to dust up the Assassin modlist I did 2 years ago: I'll be updating interesting mods for this class as well as make role-play suggestions.
This time, I'll concentrate on the Argonian Shadowscale.
Here's what the UESP says about Shadowscales:

The Shadowscales are an order of Argonian assassins originating in Black Marsh. Argonians born under the sign of the Shadow are taken at birth and offered to the Dark Brotherhood where they are trained in the arts of stealth and assassination. Any Shadowscale who lives to come of age is accepted into the Dark Brotherhood as a full member of the family.They may also leave the Brotherhood to serve the Argonian people in Black Marsh.

Character Creation

Argonian

You have three race options here. The first one is to go literally with a dark scales Argonian with Xenn's mod called Shadowscales. The textures are a bit aged for today's standards, so you might prefer to go with a sexy Argonian race replacer.

Wey's Argonians is fairly popular and draws inspiration from Skyrim's Argonians. The body texture is quite dark and works well for a Shadowscale.

My absolute favourite Argonian replacer is Red Fury Demon's. The textures are gorgeous. Personnally, I like to use the default red/orange texture for male, and the optional green texture for females. Might not be lore-friendly but I can finally tell them apart.

Birthsign and deity

Really, you should be picking the Shadow. If you find the sign a little boring, there are a couple of mod options:

• Birthsigns Are More Fun by Alaisiagae, a classic. This mods takes inspiration from the Oblivion birthsigns

Shifting Shadows (Ability): Sanctuary 5pts (new)
Moonshadow (Power): Invisibility for 60 secs (unchanged)
Nightshadow (Power): Invisibility for 120 secs (OB)
Wraithwind (Power): Fortify Sneak 30pts for 120 secs (OB)
Fingernail Moon (Power): Chameleon 15% for 120 secs (OB)

• Expanded Birthsigns for Purists by Cael_Aurion offers two variants for each birthsigns (Masser and Secunda variants)

Masser
Shadowbender: Fortify personality +10 pts., fortify illusion SKILL +10 pts.
Spell Shadowmeld: 30 pts Chameleon on self for 30 seconds, costs 15 magicka, always succeeds.

Secunda
Silent Step: Fortify Agility 10 pts, Fortify sneak SKILL 10 pts.
Power Moonshadow: Invisibility on Self, 60 seconds (once a day).

• MTR's By the Divines lets you pick a deity (and a power it grants). Sithis awaits.

Gameplay and Mods

Gear

• Black Armor Set by Kalian, because green isn't all that stealthy.
  
• Westly's dark brotherhood armor replacer: Better bodies fit, I've always liked this one.
  
• Dark Brotherhood Armor Replacer Expanded with Ranks: has different coloured suits of armor to reflect the ranks of the Assassin. The style is clearly martial art inspired. Still a good fit.
• Blade of Sithis by Gojiragamer
• Assassin's Armory: a shop located in the main Morag Tong guild hall, with quite a nice collection. Of note to your perusing Assassin, are the off-hand weapons (shields) as well as some exotic weapons (scythe, sai).

Quests

• Rejoice Caeris heard our prayers and gave us Vvardenfell Brotherhood where the player can join the Vvadenfell branch of the Dark Brotherood.
• Cherry on top, it actually ties-in with his other mod Morag Tong Polished (collab with Luj1). You can actually join both guild in a reasonably lore-friendly way.
• Wolli's Suran Underworld is a large quest mod for the darker, grittier character.
• Random missions by Teclis: a hidden gem that sends you on random assassination missions using Starfire's NPC additions. Ask Eno Hlaalu for Morag Tong Writs. You can watch a quick demo in this YouTube review of Starfire's NPC additions.
• The Bounty Hunter Bundle by Gavrilo: for the assassin eager to make more coin (legally) with his trade.

Other mods

• Corsair's Severa Magia DB fix so that Severa Magia's lair refects its affiliation with the Dark Brotherhood.
• The Assassin has little to no magic ability, so consider buying scrolls, having them made with MWSE 2.1 Enchantment Services by Svengineer99. Cantrips, is also a good way to have at least some minor magic options.
  
• Need a cosy home? Arena Cistern Lair by Bradford Horton is perfect and not too far from the Morag Tong. If you want more storage, be sure to grab Greatness7's MWSE Containers.
  
• Be classy, leave a Calling Card next to your victims. Mod by Rattfink333.

Gameplay

Sneak Mods, choose only one!

• The Lighting Mod - Modular Only use the sneak related esp. Makes sneaking harder or easier based on light, time, and weather. Faily heavily scripted.
• Thief Experience Overhaul 1.3 Only activate the stealth enhancement esp. Give the player a bonus to sneak at night and in bad weather.
• TESIII Sneaking Realism: a different approach with this one. Wearing armor and having your weapon drawn will reduce your ability to successfully sneak until you are more skilled with sneaking. Once you become more proficient with sneaking the debuff is nullified; or in some instances, you will receive a bonus to your sneak skill.

Combat mods

• Brutal Backstabbing by Merlord: the bread and butter of the Assassin; backstab your target for extra damage! Comes complete with satisfactory sound and pop up message to confirm your hit, entirely configurable in-game.
• Vituvian Guar's Sneaky Strike where the crit multiplier varies from weapon type to weapon type.
• Another excellent mod from Vituvian Guar: Nimble Armor that adds an "evasion" stat based on the armor and the skill of the player. In addition, successfully doging an attack 9thanks to your evasion) contribute to your skill progress in Unarmored ad to a lesser extent Light Armor
  

Poison and alchemy mods

Poison Crafting by Greatness7:

• PRO: icon and bottle replacers for potions, easy to identify them
• PRO: no need to pick up apparatus (like the classic "At Home Alchemy"
• CONS: you need to re-apply the poison after each strike

4NM_Alchemy

• PRO : new icons for bottles
• PRO: implements a "charge" system so you don't need to re-apply poison as often.
• PRO: no need to pick up apparatus (like the classic "At Home Alchemy"
• PRO: you can also throw the bottle of poison
• CONS: comes bundled with an anti-potion exploit: you cannot drink more than a potion at once AND based stats are used when making potions (so forget about cheesing your fortify intelligence)

Poison Redux-ion by OEA:

• PRO: mostly based on Poison Craft and includes most of its features
• PRO: when a weapon is coated with poison, the effect is added to the weapon's enchantment.

Alchemical Knowledge by Vituvian Guar, the best potion crafting and alchemy overhaul we have right now, in my humble opinion. Taking inspiration for Skyrim, eating an ingredient teaches you its first effect (In Morrowind, you know the effects based on your alchemy skill). In addition, the alchemy UI has been greatly improved: like in Skyrim, you select an effect, and known ingredients with that effect will be shown.

Marksman

I prefer my Assassins to get up close and personnal with their target but if you prefer ranged attacks, there's a couple of mods that you should get:

• Pincushion because you want to see that arrow you shot right in the eye of your target.
• Merlord's Automated Ammo Equip because you don't want to waste time. Alternatively use Smart Ammo by Abot, it has a few more features.
• Marksman Overhaul Hitting a target at a greater distance is harder, crouching/sneaking improve the odds. (MCM menu to tweak those odds to your liking)

Handicaps and goals

Handicaps

• Never get arrested for murder.
• Only use alchemy for poisons.
• Illusion spell restriction: blind, chameleon, invisibility, night-eye, paralysis and silence only.

Goals

• The Shadowscale does not need to serve the Dark Brotherhood if he serves Argonia. Be on a mission to free Argonian slaves Free the Slaves by Caeris, or even abolish slavery with Brother Juniper's Twin Lamp. Track and hunt down Camonna Tong, especially with Strike at the Camonna Tong

For a full list of slave related quests and mod, see my Escaped Slave Challenge. I did a let's play as well (main mods: 4NM series, OJ's deleveller)

• Strive to undermine the Tribunal, the power behind Vvardenfell, weaken Morrowind before Argonia rises against the oppressors: completing the main quest with a bit of god-killing action should do the trick). For a real challenge facing Vivec, try Versus Vivec by Trainwiz.
• If your main allegiance is the Dark Botherhood, make it your mission to track and destroy the Morag Tong.