The Lost Levels

The Lost Levels is a second level campaign unlocked after beating the original campaign on Doom 64 (2020) port or by beating the super secret level Hectic. It is intended to link events of Classic Doom series into events of Doom (2016)/Doom Eternal.

Background
The Lost Levels are a separate campaign that unlock after you beat the game or complete the super secret level “Hectic”. The level maps in the Lost Levels are brand new to Doom 64. Once reaching the end game character reel, return to the main menu and start a new game.

Plot
Following defeating the Mother of All Demons, Our Hero spent time tearing up Hell. The Mother's sister Resurrector kicks the Marine out of Hell back into a base on Earth, and tries to kill him. He fought his way through the base to find a portal back to Hell. He fights his way through this new section of Hell, and takes revenge on the sister. He again chooses to fight for eternity in Hell (according to developers linking Doom 64 to Doom (2016)/Doom Eternal).

Ending
You had not expected to be torn from Hell so soon after your fateful decision. Getting back there was your only concern. The plans of the sister Resurrector to exterminate you have failed.

A grim vision takes hold of your mind as the demon carcasses steam in your wake. Stretched before you is a path of perpetual torment... A path through DOOM...

Levels

 * MAP34: Plant Ops
 * MAP35: Evil Sacrifice
 * MAP36: Cold Grounds
 * MAP37: Wretched Vats
 * MAP38: Thy Glory
 * MAP39: Final Judgement

Fun Level

 * MAP40: Panic

Behind the scenes
The Lost Levels is an all new bonus campaign for Doom 64 in the vein of Doom 3: The Lost Mission, and No Rest for the Living (Doom II), and Thy Flesh Consumed (Doom).

In addition a scene linking Doom 64 and Doom (2016) can be seen in Doom Eternal. When Marine is discovered by the priests on Argent D'Nur, they hold his classic Doom helmet (particularly the version seen in the end game cutscene of Doom 64), that he was wearing while he fought in Hell.

Strangely, the fact that the Mother Demon has a sister seems to contradict the fact that she is supposed to be the mother (or at least ancestor) of all demons. However it may be possible that by sister, it means that she also has the power to resurrect demons or is same demon subspecies as the Mother Demon, rather than being a biological sister