Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

Vexedart has a deathsinger (Lyra’ssandra or something) that I’ve run through epic content with on several of my characters. Practically untouchable, does solid damage, great with group buffs as well. One of the stronger characters I’ve run with.

The UD pit fiend, which gives me more trouble than the surface pit fiend or the Balor, can’t touch her.

I have no idea what her build is but I’d ping Vex on discord for advice. Definitely knows how to build and play one on the server which in my mind is better than just theory crafting a build because the environments you’re actually playing in often effect what skills and/or abilities you might value over others.

The biggest issue with the split Wizard (10), Bladesinger (10), Dragonslayer (10) is getting Dispelled. With a 26 CL, toons will suffer from Greater Dispel often. This is not a theory, it is the truth: I've played a number of 25-26 CL builds this last year and any Boss or mob with Greater Dispel is destroying my builds.

The Wiz10/BS10/EK10 is the best bet, since with only 5% change of dispel, your toon will last. As I noted above. It is easy to build this out in NWN2db to see for yourself.

You can even build a silly Swashbuckler 7 / Duelist 10 / Wiz 3 / Deathsinger 10, not even cast spells, and get:

AC:
21 + 2 (with Cat's) + 10 INT + 6 IMA wand + 4 shield spell + 5 Defl. + 4 Natural + 4 Dodge +1 One Weapon = 57 (+ 1d4 One Weapon; +3 Combat Expertise; 60 AC in most instances; 50 AC if wand magic dispelled)
AB:
34 + 2 (w/ Cats) + 4 EB + 4 Gr. Heroism = 44 (-3 CE + 1d4 One Weapon = 41-45)
DMG:
1d6 + 4 EB +3 STR (w/ Bulls) + 10 Combat Insight + 2d6 Precise Strike + 10 Insightful Strike = 36 avg (less against crit immune)

The Death/Bladesinger is a small needle to thread, and thus it isn't some PrC that is going to bring a lot of variety among its Death/Bladesinger peers.