Ideas for a thief/swashbuckler.

[Korvax]

what feat is it, and where is it found?

Daring Outlaw is the feat, and it's found in Complete Scoundrel. I'm currently playing a swash5/rog3 and having a ball since getting that feat (of course, CS only came out after we had started the campaign, so my build is suboptimal, but still great fun)

I would not recommend the spring attack route unless the rest of the party simply does not provide good flanking partners. Two weapon fighting is the way to make this a blast.

 

[Korvax]

Weapon Proficiency: Whip - because who doesn't like Zorro. And you should be able to use it to assist swings, jumps, etc...if you have a good DM.

A whip is another very interesting idea that could prove to be a lot of fun.

 

[Darklone]

We have a Daring Outlaw in our pirate group as well, he's using Dual Strike to great effect. TWF rocks with sneak attack AND INT to damage. (Dual Strike allows only to add sneak attack once, but it's more often useful than Two Weapon Pounce)

Since you have 10 levels to play with, I'd go ahead to build a real killermachine with ftr4/swash3/rog3 with Daring Outlaw and Daring Warrior. That gives you enough feats to build a real workable TWF fighter. Sneak attack suffers a bit but Melee Weapon Mastery makes up for it. Doublewielding kukris (low weapon damage offset by Int bonnus to damage, WS, MWM and a bit magic) with Telling Blow, Improved Critical and funny things like Two Weapon Pounce.

Two levels more and you may take Two Weapon Rend.

So. 3 feats from fighter levels, 1 human bonus and 4 feats from leveling. TWF, ITWF, Telling Blow, WF, WS, MWM, Imp Critical and now one feat is missing for Daring Outlaw and Daring Warrior... Would a trait/flaw from UA be possible to take another feat? Or get two keen kukri and skip Imp Critical.

Otherwise stick with Daring Outlaw only, go rog4/Swash6 (You'll want Uncanny Dodge!) and take Daring Outlaw, Telling Blow, TWF, ITWF, Imp Crit. Same about Imp Crit as above, you could take Two Weapon Pounce or Dual Strike instead.

 

[Unkabear]

I would suggest if your DM is game to use the Swashbuckler varient from the Living ENWorld listed here down under the additional approved material under Complete Warrior about halfway down the thread.