Which two sons of Aaron died after offering unauthorized fire before the LORD?

Answer: Nadab and Abihu

Nadab and Abihu offered unauthorized fire, and fire came out from the presence of the LORD and consumed them.

“Now Aaron’s sons Nadab and Abihu took their censers, put fire in them and added incense, and offered unauthorized fire before the LORD, contrary to His command. So fire came out from the presence of the LORD and consumed them, and they died in the presence of the LORD.”— Leviticus 10:1-2 (BSB)