Who was Eleazar?
Eleazar was Aaron's third son by Elisheba, Amininadab's daughter, descended from Judah through Pharez (Exo. 6:23,25; 28:1; Gen. 38:29; 46:12; Ruth 4:18,20). On the death of Nadab and Abihu without children (Lev. 10:1; Num. 3:4) Eleazar had the oversight of the chief Levites, who kept the charge of the sanctuary (Num. 3:32).
With Ithamar his brother he ministered as a priest in his father's lifetime, and was invested in Aaron's highpriestly garments as his successor, on mount Her, just before his death (Num. 20:25-28).
With Moses he superintended the census (Num. 26:3), inaugurated Joshua whom Moses set before him (for Joshua was in this inferior to Moses, who had direct intercourse with God; Joshua must ask divine counsel through the high priest), and divided the Midianite spoil (Num. 27:22; 31:21).
He took part in dividing Canaan (Josh. 14:1). He was buried in "the hill of Phinehas his son, ... mount Ephraim" (Josh. 24:33). The high priesthood passed to Ithamar's line in the person of Eli, but for the sin of Eli's sons reverted to Eleazar's line in the person of Zadok (1 Sam. 2:27; 1 Chr. 6:8; 24:3; 1 Kings 2:27).
IBSE, "Isaac" (in the public domain) with minor edits.