Overlord is the final level within the second campaign.
You start underwater - swim forward, and when you reach the chamber, tuen right and look for the shotgun in the corner. You can also look for a bloodied handprint on a pillar opposite of the start, to reveal an atomic health in the center pillar.
Surface, and clear out enemies. In the raised section, you will find four buttons. Press the leftmost button, and the two right-most button to lower the center pillar - giving access to the freezethrower by a long jump.
If you have the jetpack from the first stage or another source, you can break the vent in the chamber and head directly to the boss fight.
Without the jetpack, there is only one reachable exit from this area. Open the door. You can open the picture showing Saturn to collect some health and ammo. Behind the corner is a battlelord, which can be (barely) killed with the weapons you already collected.
Past the battlelord is a large room. To ascend the room, head to the right and climb the ramp. Repeat on the opposite side to get to the top of the room. The far corner has a switch that you may flip - this removes the forcefield in the ventilation.
Backtrack until you see the vent cover, and break it open. This leads to a shaft, which you will have to jump down to reach the boss. The other two passages lead to the starting room, and may be skipped.
The arena starts with explosive canisters and Protozoid Slimer eggs. The eggs will hatch - clear them out first, as they can deflect rocket fire you make at the boss, causing a suicide.
You can also check the side passage, swim underwater to collect the Devstator. The side passage also leads back to the second large room, described previously.
When ready, approach the large door to reveal the Overlord. It only has one attack, a rapid fire RPG, but like the previous boss, he will instantly kill you if you get too close. The rockets are easily dodged by sidestepping and circling the enemy. If you need additional ammo to defeat him, there is some RPGs in the command center, as well as a secret area.
- In the first room, the secret is located under the raised section below the four switches. To reach it, swim towards a concealed passage just under one of the ramps, while on the water's surface.
- When you reach the freezethrower, you will open a secret door for a few seconds. From the freezethrower platform, continue jumping across the water and towards the pillar. The side of the pillar facing the vent contains a secret. It contains an RPG.
- In the second large room, the first cooridor tat leads to the second level has a cracked wall. Blow it up to reveal pipebombs.
- In the boss chamber, head to the command center behind the spawning point of the overlord to open a door; you only need to cross into the expanding section for the door to open. Once you do so, run to the opposite side of the arena to collect additional ammo.