What I want to do is create a new character FROM SCRATCH. He will be an Imperial Swordsman and assassin. Name: Lucius. In the case of assassins, my character WILL NOT join the Dark Brotherhood. More of a neutral assassin than evil in the sense that he doesn't kill innocents and rather kills corrupt people or generally people who the world would rather do without. People like bandits, corrupt Counts, and Imperial Legion Captains that abuse their power.

My plan is to not use magic but rather use bows. But bows will also be rarely used, only to kill long ranged targets.
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Personality wise, my character never kills innocent people. Rather, he goes and kills corrupt politicians He is not bloodthirsty and therefore, goes in, kills efficiently and goes out. No ritualistic ceremony and believes he's the hands of the Nine Divine. Has strong faith in Akatosh and Talos especially. Not very materialistic and asks for very little. However, he does loot the houses of his victims and gets money that way.

Wants to help innocents if possible, especially if helping them involves Lucius' skills.
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Gameplay wise, I want my character to sneak around a lot and not kill everything in sight but rather sneak around people, get crafty by luring enemies into their own traps, etc. It's extremely fun and I'd like to do something other than pure battle. If I want to do a pure battle, I'd play assassin's creed brotherhood and mess around there.

Guilds I'll join will be the following and the explanation why my character would.

Thieves Guild: Doesn't involve killing people who are necessary and helps those who actually need it. Not to mention it's a good way to practice sneaking around and getting good money.

Fighter's Guild: Obviously. Lucius is a very good fighter so it'll be pretty good for Lucius. He'll probably stay at a regular guild member and not advance up in the ranks. Maybe assassinate some Blackwood Company people after discovering that they use Hist sap.
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So what should my skills and birthsign be?

Role play build-

Marksmen
Sneak
Secruity
Light Armor
Mercantile
Speechcraft
Illusion

Birthsign- Warrior, Thief or Shadow (take your pick)

This build is miles away from a good build (ie efficient leveling) but is fun for RP.

Are you planning to wear leather and furs or sneak a look at the DB so you can use the Shroud Armor? Oblivion does not give a good choice of apparel for a high level warrior-thief.....