There are some universalists who believe that Satan and all of the demonic realm will be redeemed through the atonement of Jesus. Though I have found this is a minority view among the universalists, it is still worth addressing and correcting.
The demonic realm will not and cannot be redeemed. This simple reason is that they have no Redeemer. Jesus is the Redeemer of humanity, not of the demonic realm.
There is a doctrine in theology called the hypostatic union. It is the doctrine of the two natures of Jesus. Jesus is both God and man. He was God so that He could appease God the Father with the sacrifice of infinite value. We need a sacrifice of infinite value because when we sin, we sin against an infinite God. Therefore our offense against Him has infinite consequences.
No mere human can please and satisfy an infinitely holy and righteous God. Only God Himself can satisfy the infinite requirements of His own holiness. Therefore, "the Word became flesh and dwelt among us," (John 1:14). Jesus became our atoning sacrifice to turn away the wrath of God (Rom. 5:9) by bearing our sins in His body on the cross (1 Pet. 2:24). Therefore, those who trust in Christ are saved by grace (Eph. 2:8-9; Rom. 6:23).
Likewise, the reason he needed to be a man was so that he could atone for the sins of mankind. He had to be made like one of us.
"For assuredly He does not give help to angels, but He gives help to the descendant of Abraham. 17Therefore, He had to be made like His brethren in all things, that He might become a merciful and faithful high priest in things pertaining to God, to make propitiation for the sins of the people." (Heb. 2:16-17).
The fallen angels do not have a sacrifice on their behalf. They do not have the infinite God becoming one of them and atoning for them. Therefore, the demonic realm will not be saved.
- John 1:14 - "And the Word became flesh, and dwelt among us, and we beheld His glory, glory as of the only begotten from the Father, full of grace and truth."
- Rom. 5:9 - "Much more then, having now been justified by His blood, we shall be saved from the wrath of God through Him."
- 1 Pet. 2:24 - "and He Himself bore our sins in His body on the cross, that we might die to sin and live to righteousness; for by His wounds you were healed."
- Eph. 2:8-9 - "For by grace you have been saved through faith; and that not of yourselves, it is the gift of God; 9not as a result of works, that no one should boast."
- Rom. 6:23 - "For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord."