I'm looking for a second sword to back-up my Nightblade. What's a better choice?
Flourish or Troika
alternatively, make a Leviathan blade, or a Cold Iron Vanquisher. high normal damage against everything, CIV has increased damage vs undead.
3 hit combo, same kinda speed as a nightblade, so you wont have much adjustment of your playstyle to do.
Flourish, hands-down. You only need two damage types to hit everything for at least neutral. I wouldn't go for Leviathan, since I feel like you only carry that if it's the only sword you'll be using, not to complement anything else.
I actually use a Troika (ow a Kamarin) and I find that it is pretty good (for me). I also have a snarble bard which is sort of just like a flourish and I use it often. They're both okay, so I would recommend a Troika and a Flourish. If you can only have one though, get the Troika. It can deal normal damage, which nothing is resistant to, and it isn't as slow as everyone thinks.
Well since the Troika only upgrades to a maximum of 4*'s I'd go with the flourish. Also, since the flourish line does piercing damage it would help deal with the creatures resistant to your Nightblade (Fiends to whom it does extra damage and Undead to which it is neutral). If you prefer the slow feel of the Troika I recommend the Sealed Sword line upgraded to the Divine Avenger for elemental damage.