Numbers 16 - 18
Korah incites a rebellion against Moses and Aaron, followed by Dathan and Abiram, and many others. See 2 Timothy 3:1-9, Jude 1:10-11, and other passages regarding those who seek to elevate themselves against God’s Will.
God judges all who rebelled, including some 14,000+ of the people who also complained against Moses and God, but Moses and Aaron intercede on behalf of the people, and God relents.
The Lord makes it clear that Aaron is His choice as High Priest, and calls out the blessings that will be upon the priestly classes for their service to God.