Nadav and Avihu were the sons of Aaron. Hashem took away their souls when they brought offerings into the Holy Tabernacle out of schedule, and other reasons. (Leviticus 10:1-10)
The souls of Nadav and Avihu were souls of priests on the highest level. They were kept out of the correction process, waiting for Pinchas to receive them. As we learned previously, Pinchas stopped the death from spreading in the camp of Israel. After he killed Zimri and Kozbi which caused the situation, his soul left his body. The two souls of Nadav and Avihu entered his body and earned him immortality and eternal priesthood inherited from his uncles, Nadav and Avihu.
Read more here: https://dailyzohar.com/daily-zohar-1801/

Synonyms:
Nadab and Abihu