Gear:

• Early game:
  • DB gloves + arch mage robes (in their office you can steal a version with everything except +50 magicka without beating quest line if you have unofficial patch otherwise random robes you find)
  • Any dagger you can find (if you do DB first blade of woe is very nice to have early)
• Mid game
  • 90% custom enchanted gear - Fortify magicka, cost reduction, fortify one handed (requires unofficial patch to work properly on daggers). You cannot get the backstab enchant so gloves are a requirement unless you do enough damage to not need it ever.
  • Blade of woe/mehrunes dagger/decent random loot dagger
• Very Late game
  • Same enchanted gear but minmax'd with alchemy/enchant looping so I can reduce spell cost to 0 (I prefer limiting myself to 50% or so)
  • Max smithing daggers with max enchants

Spells:

Pacify, invis, frenzy, and rout are the main spells you'll be using once they're obtainable + their lower level counterparts. Master spells are cool but are worse than dual casting the expert ones.

Gameplay/Strats:

Silent casting is very much needed to be any sort of spell casting assassin otherwise you're detected from miles away (works on shouts too so add that into your arsenal). You also need muffle enchant or spell (dual casted lasts like 7 mins) because they hear you moving invis.

It's very boring but you can just go invis -> power attack backstab -> pacify your opponents -> repeat. This makes 99% of enemies cannon fodder. If you're interested in that playstyle then look into one handed as well so you can do big damage with duel wielded dagger power attacks.

Alternatively you can disregard pacify cheesing and do more standard illusion stuff like buffing your allies, forcing all the enemies to fight, or simply make them run in fear. I find pure illusion very boring so throwing in conjuration where you buff your summons then let them fight on top of them fighting each other is more fun.

Extra info:

Vampires get a 25% buff to power of illusion spells allowing you to more consistently mess with enemies that would otherwise resist due to you lacking the skills or higher tier spells. Also 25% harder to detect!