Jude 1:6 And the angels <angelos> which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.

Peter speaks of the same event thus:

2 Peter 2:4 For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;

In the New Testament angels does not always refer to angels from heaven. It is also used of men. Below are some places where it is so used.

Matthew 11:10 For this is he, of whom it is written, Behold, I send my messenger before thy face, which shall prepare thy way before thee.

Luke 7:24 And when the messengers of John were departed, he began to speak unto the people concerning John... 27 This is he, of whom it is written, Behold, I send my messenger before thy face...

Luke 9:52 And sent messengers before his face: and they went, and entered into a village of the Samaritans, to make ready for him.

James 2:25 Likewise also was not Rahab the harlot justified by works, when she had received the messengers , and had sent them out another way?

Revelation 1:20... The seven stars are the angels of the seven churches: and the seven candlesticks which thou sawest are the seven churches.

Revelation 2:1 Unto the angel of the church of Ephesus write...

The angels referred to by Jude and Peter are Korah Dathan and Abiram. These were men who held prominent positions in Israel but were not satisfied with their allotted role (their first estate) and made an attempt to usurp the positions of Moses and Aaron. The event is recorded in Numbers 16.

Numbers 16:1 Now Korah, the son of Izhar, the son of Kohath, the son of Levi, and Dathan and Abiram, the sons of Eliab, and On, the son of Peleth, sons of Reuben, took men: 2 And they rose up before Moses, with certain of the children of Israel, two hundred and fifty princes of the assembly, famous in the congregation, men of renown: 3 And they gathered themselves together against Moses and against Aaron, and said unto them, Ye take too much upon you, seeing all the congregation are holy, every one of them, and the LORD is among them: wherefore then lift ye up yourselves above the congregation of the LORD?

The fate of these men was a notable event to serve as an example for others.

Numbers 16:23 And the LORD spake unto Moses, saying, 24 Speak unto the congregation, saying, Get you up from about the tabernacle of Korah, Dathan, and Abiram. 25 And Moses rose up and went unto Dathan and Abiram; and the elders of Israel followed him. 26 And he spake unto the congregation, saying, Depart, I pray you, from the tents of these wicked men, and touch nothing of theirs, lest ye be consumed in all their sins. 27 So they gat up from the tabernacle of Korah, Dathan, and Abiram, on every side: and Dathan and Abiram came out, and stood in the door of their tents, and their wives, and their sons, and their little children. 28 And Moses said, Hereby ye shall know that the LORD hath sent me to do all these works; for I have not done them of mine own mind. 29 If these men die the common death of all men, or if they be visited after the visitation of all men; then the LORD hath not sent me. 30 But if the LORD make a new thing, and the earth open her mouth, and swallow them up, with all that appertain unto them, and they go down quick into the pit; then ye shall understand that these men have provoked the LORD. 31 And it came to pass, as he had made an end of speaking all these words, that the ground clave asunder that was under them: 32 And the earth opened her mouth, and swallowed them up, and their houses, and all the men that appertained unto Korah, and all their goods. 33 They, and all that appertained to them, went down alive into the pit, and the earth closed upon them: and they perished from among the congregation.

Normally burial in the earth is no more than 6 feet or so, but in the case of these men the earth opened and they were buried a long way down.

Peter corroborates this by using the word ‘tartarus’ which is translated hell.

‘cast them down to hell

‘Tartarus’ in Greek mythology is a place in the underworld far below ‘Hades’. It is as far below earth as earth is from heaven.

It is the only time ‘tartarus’ is used in the New Testament. Peter didn’t believe in Greek mythology so is using it as a figure to describe a descent into the earth deeper than the normal burial of the dead.

Korah Dathan and Abiram sleep the sleep of death, deep in the earth, awaiting the day of resurrection and judgment. This is described as ‘reserved in everlasting chains under darkness unto the judgment of the great day’ by Jude and ‘delivered them into chains of darkness, to be reserved unto judgment’ by Peter.

I hope this helps.

God bless you,
Glenn