Cycloid Emperor is the leader of the alien race Duke battles. Encountered in Stadium, it attacks the player with a rapid-spread version of the Octabrain's mental blast at close range. It will also fire a large barrage of missiles at a distance, with the exact amount of damage inflicted as Duke's RPG.
There are also two other Cycloids, the bosses of Duke it out in D.C. and Duke Caribbean: Life's A Beach.
The Cycloid in Duke it out in D.C. is the same, with only difference that the battle arena is smaller, making the fight harder. The Cycloid in Duke Caribbean: Life's A Beach has a different model (see Gallery below), as he wears Caribbean attire, and also attacks Duke from behind.
The Cycloid Emperor is taller than the Battlelord and Overlord and can be recognized by its single-eyed "face" and huge tripod claw arms. Unlike the Battlelord, the Cycloid Emperor doesn't have a real physical weapon; it uses its claw arms as a barrel for the missiles. Like the Battlelord however, it screeches horrifyingly and standing idle looking at this monster is mostly paid for by dying.
It has 4500 hit points and is unaffected by the Shrink Ray. The Cycloid Emperor ranks as one of the most dangerous of the bosses in Duke Nukem 3D due to the erratic nature of its main attack which fire rockets in salvos of 2-3 per shot all off which are thrown out in unpredictable angles. The Cycloid Emperor's other attack, the mental-orb spray is a lot more manageable, but it usually does this only at short to medium range to prevent damaging itself with its own rockets. It should be noted the Cycloid Emperor will immediately fire rockets if the player hits him with any weapon; consequently, the Emperor may not have the opportunity to use the Mental Blast against the player.
He was the final boss in the original version of Duke Nukem 3D but in the Atomic Edition a fourth episode was added introducing the Alien Queen which makes him the second-to-last boss from v1.4 onwards.
When Duke first encounters the Cycloid Emperor, he'll utter "It's down to you and me, you one-eyed freak!". Upon death, Duke will say, "Die, you son of a bitch!". The corpse of a boss cannot be destroyed, i.e. gibbed, compared to the common enemies encountered in the game, and will remain so until the level ends. The same dying animation will show regardless of what weapon the player used to defeat a boss.
A Cycloid Emperor in maps with the default palette of 0 will only be able to walk on sectors with a Lotag of 3. This is why it cannot leave the field in Stadium, and why in most stock maps that use one it cannot move from its initial spot upon activation. This behavior does not happen to Mini Cycloid Emperors given any other palette.
As with the Overlord and the Alien Queen, basic programming was just copied from the Battlelord, and there is a leftover of a Mini Battlelord in the Cycloid Emperor. The Cycloid Emperor has the same appearance as the Mini Battlelord while in another palette, it has 1 hit point and will die instantly. Also, the rockets launched from the Cycloid Emperor don't spawn correctly and thus will kill it.
Interestingly, if a non-palette Cycloid Emperor is artificially given a smaller size, this phenomenon doesn't occur.
Due to the fact that the Cycloid Emperor with the default palette of 0 can only move in an area with a lotag of 3, some fan made maps have actually been able to turn this into an advantage. A notable example includes the final level in the fan made episode Total Apocalypse; in this level the Cycloid Emperor is protected by a moving barrier that makes it a lot harder to hit since the player will often end up hitting the barrier instead. Because the Cycloid Emperor can't move it won't suffer splashback damage if any of its own missiles hit the barrier. Also, Paletted 3 Cycloid Emperor was a same size as its boss counterpart can move in a area rather than zero.
The enemy Cycloid was shown in the playable demo for Duke Nukem Forever shown at PAX in Seattle in September 2010. Like in Duke Nukem 3D, the fight occurs in a stadium. After defeating it, Duke can elect to kick its eyeball into the field goal.
In Duke Caribbean: Life's A Beach the Cycloid Emperor makes an appearance as the final boss with a Hawaiian outfit to fit the general Caribbean theme of the game.
In Duke Nukem: Total Meltdown the Cycloid Emperor looks similar to the original one found in Duke Nukem 3D in shape and size, however its contrast is a lot darker giving it a much more menacing look.
In the Duke Nukem 64 port, it is rendered with polygons and is the only full-3D boss in that game. Furthermore, it is larger and slower than the original one while also having more health, thus it can take a lot more beating. It attacks similarly to the Duke Nukem 3D Cycloid Emperor. It shots rockets (although with the left arm) and a rapid mental blast at close range. However, at a third of its health it will replace the rockets it fires with half-charged plasma bolts.
In Duke Nukem: Zero Hour, the final boss, Zero, appears to be a Cycloid himself, being as tall as the Cycloid Emperor, having a similar appearance and a single eye. Zero uses artificial wings in combat, and has a few types of laser attacks.
In Duke Nukem: Land of the Babes, the final boss, Alien Slave Boss, who controls the Earth occupation in the future, appears to be a mutated Cycloid. He sports a similar appearance with one eye, only being much smaller, having green skin, and piloting a flying machine.