The main focus is on Illusion and Sneak. Being a vampire boosts both these skills by a whooping 25%, making you the best stealthy character in the game. Since the master difficulty setting has a tendency of one shotting you anyway (if you don't go all out on health level ups) I decided to abandon health all together and rely on the shadows and cunning tactics to keep me alive. As a result you'll only have 40 HP left when your vampirism reaches stage four during day light.

The way to dispose of enemies is mainly by means of sneak attacks and illusion magic tricks. If there's a room full of enemies let them first fight each other (Fury/Frenzy/Mayhem) till there's only one or a few left and then sneak attack the remaining (damaged) victors.
I've decided to do this with dual wielding maces instead of daggers. The sneak attack factor is obviously less (6vs15 or 12vs30) but the maces more than make up for this because they provide the highest base damage. This can be boosted even further by fortify one-hand enchants (unlike daggers). Plus they ignore 75% armor making them more reliable against all enemies.
The difference is that enemies detect you after a stealth kill (unlike daggers). However, I consider this an upside regarding gameplay though because it adds a certain risk to the game, whereas with daggers you can kill a whole room without the dumb AI reacting one bit, which is not very realistic at all.
With this type of gameplay you have to combine multiple combat styles which increases the tactical aspect of the game.
Late game the two enchants you'll want are fortify sneak and one-handed. This means you won't have zero mana spells but that's no problem since you didn't need to invest in health level ups anyway. Having to be careful with your mana bar adds another tactical aspect to the gameplay since you might be overun without mana left for your bag of magic tricks. As a result; the preferred racial choice is High Elf for the constant +50 mana and mana regeneration spell. All other options are somewhat useless because resistances don't matter if you only have 40 hp anyway. Orc could be considered for the once a day temporary double damage but High Elf benefits far outweighs it imo.
The constant need to feed as a vampire can be annoying in 'normal' games. With this playthrough I decided to stay in the final blood starved phase as much as possible. This means you'll have to sneak around in towns and calm (illusion) the people you want to talk too.
Kinda like in the game "Vampire: The Masquerade" where playing as a Nosferatu you had to go through the sewers to go places because you were so hideously deformed and didn't look like a human anymore.
In the 4th and final Vampire phase you'll have certain dis- and advantages:
Pro:
+ Free spell: Embrace Of Shadows (Invisible with improved night vision for 3 minutes per day)
+ Free spell: Vampire's Servant (Reanimate a very strong dead body for 60 seconds per day)
+ 100% frost resistance. (Not sure if it is an actual 100% immunity or is also capped at the standard 85%. If not capped then this benefit is massive since your 40 hp suddenly becomes infinite against frost attacks.)
+ 100% resist decease and poison. Awesome
Cons:
- 100% fire vulnerability. Normally it's really bad but with this 40hp build... who cares. They won't/can't hit you anyway.
- During daylight (from 5 am to 7 pm) you have minus 60 hp, magicka and stamina and -100% regeneration. The regeneration is difficult and makes dayfights more difficult where you have to manage your massive mana reserves more carefully. Gameplay side effects may include: happiness to go inside.

- As aforementioned; people don't like you very much, which is an understatement.
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All in all being a dedicated vampire assassin means you'll have to embrace the darkness to the fullest extend and accept your new way of life in this lonely world full of vampire racists. :ninja:
Instead of seeing the feeding as a 'mandatory' nuisance; reap the amazing vampiric benefits and roleplay a proud vampire who accepts his final form.
Appreciate your thoughts & ideas.