Ultimate Infiltrator (3.5e Optimized Character Build)
From D&D Wiki
I was thinking it would be a lot of fun as a player to infiltrate an enemy base using magic. Then I thought about making the coolest possible level 20 infiltrator with SRD rules only. Here he is!
 Game Rule Components
None are nessessary. A true infiltrator uses any tools available!
Race (Templates): Human
Starting Racial Traits: Extra Skill Point, Bonus Feat
|Saving Throws||Feats|| Class
|1st||Rogue 1||+0||+0||+2||+0||2 feats||Sneak attack +1d6||Disguise (4), Hide (4), Move Silently (4), Disable Device (4), Decipher Script (4), Escape Artist (4), 20 skill points for other skills|
|2nd||Cleric 1||+0||+2||+2||+2||Trickery Domain, another fitting domain||Knowledge (Religion) (5)|
|3rd||Cleric 2||+1||+3||+2||+3||silent spell||Knowledge (Religion) (6), Hide (6)|
|4th||Cleric 3||+2||+3||+3||+3||+1 int||Hide (8), Move Silently (5)|
|5th||Wizard 1||+2||+3||+3||+5||scribe scroll, familiar||Knowledge arcana (6)|
|6th||Wizard 2||+3||+3||+3||+6||feat||Move Silently (8)|
|7th||Wizard 3||+3||+4||+4||+6||Decipher Script (7), 3 skill points for other skills|
|8th||Mystic Theurge 1||+3||+4||+4||+8||+1 int||6 skill points for any skills|
|9th||Mystic Theurge 2||+4||+4||+4||+9||feat||6 skill points for any skills|
|10th||Mystic Theurge 3||+4||+5||+5||+9||6 skill points for any skills|
|11th||Mystic Theurge 4||+5||+5||+5||+10||6 skill points for any skills|
|12th||Assassin 1||+5||+5||+7||+10||feat||+1d6 sneak attack, death attack, poison use, +1 int||Disable Device (7), Escape Artist (7), 3 skill points for other skills|
|13th||Arcane Trickster 1||+5||+5||+9||+12||Ranged legerdemain 1/day||Disable Device (7), Escape Artist (7), 3 skill points for other skills|
|20th||Arcane Trickster 8||+9||+7||+13||+16||Ranged legerdemain 2/day, Impromptu Sneak attack 2/day, sneak attack +4d6||lots of skill points!|
At level 4 you are pretty good at infiltrating already. You can cast Silence and Invisibility to get into places unnoticed. In the end, however, you are basically Solid Snake. You can go in with Greater Invisibility while silenced and Nondetection. You can also memorize your spells as Silent spells so that you can cast them from inside your silence bubble. You will always get 6d6 sneak attacks because you are invisible and you can even cast dispels as a 15th level wizard to remove spells that allow enemies to see you. You can teleport if you need to and go ethereal if you need a better silent/invisible combo.
 Munchkin-Size Me
You could trade 1d6 sneak attack damage and an impromptu sneak attack (plus a bunch of feats) for 2 levels of Archmage. This could boost your caster level to 17 or use mastery of shaping to shape the silence spell to be a 5 ft. cube around you!
Edit: If you are worried to death about being seen, the Competition Domain might work for you, since it gives you a bonus on opposed checks, and almost everything this guy does is an opposed roll.
You have to be evil and the 15 int goes a long way...
 DM Counters
This build mostly looks like fun. It isn't really overpowered. Arguably the mystic theurge levels makes it underpowered.