will Szeth be able to use a normal shard blade?

[Kalaksbreath he/him]

will Szeth be able to use a normal shard blade? Or will nightblood prevent him from using his spren as a shardblade?

[RShara she/her]

He should be able to use a Shardblade. Nightblood's "bond" with people is pretty shallow, so I don't see it interfering with summoning his spren as a Shardblade. The only problem I can foresee is if he summoned them both at once and didn't have enough investiture to feed Nightblood. That might hurt the spren, but it's hard to say, since the spren isn't really the same being as Szeth. But Nightblood was able to feed on Lift just by her holding on to Szeth. So it's difficult to know for sure.

[Weltall]

He's sworn the Third Ideal of the Skybreakers which means he has a spren that can be summoned as a Shardblade in theory. I suspect however that the spren is very nervous around its Investiture-eating counterpart so whether it wants to come out and play in the Physical Realm is probably a bigger obstacle than anything Nightblood might do.

Mechanics-wise, there's nothing we know of that prevents Szeth from using both at once. Nale can use his honorblade and (living) shardblade at the same time and has a bond with each, the former being relatively shallow like the bond Nightblood forms with people. While Nightblood will consume nearby Investiture sources when drawn and a spren is going to look mighty tasty, it's probably going to be the last thing he goes for other than Szeth's own innate Investiture so as long as Szeth has any other form of Investiture to feed him it shouldn't be a problem.

Edited October 11, 2019 by Weltall

[Wander89 he/him]

I would imagine there would be no bounds for Szeth to do so. Nale uses his Shardblade and Honorblade. I think it's a question of whether Nightblood would consume the Investiture of the Shardblade.