The Master used a fish-shaped submersible vehicle (now destroyed)
which had these stats:

One day he fell under a hypnotic command to turn around and journey north again. He did so until he came to the source of the command, an immense starship. This was a colonization ship of the alien Plodex. The Plodex method of conquest was to send a colonization starship, containing no adult Plodex but millions of Plodex eggs, to a world whose dominant life form was in the early stages of cultural development. The starship would land in an inhospitable area, rebuild itself into a massive complex, and broadcast a hypnotic signal to cause the planet’s native life forms to come to the ship. The most intelligent creature to survive the journey to the starship would be captured and analyzed. The computer would imprint the chosen life form’s genetic pattern on the millions of Plodex eggs and launch them all over the planet.

In this case, however, one of the starship’s drive units had exploded, causing it to crash on Earth. Damaged in the crash, the starship launched all of its eggs before it had captured a specimen of the dominant native life form. Nevertheless, the starship still rebuilt itself into a colonization complex, broadcast the hypnotic signal, and captured the exile when he arrived. For days the exile’s flesh, muscle, and tissue were peeled away and his bones pulverized as the complex studied him. The pain was so great that the still-living exile went insane. Only his brain was kept alive by the starship, which incorporated it into its computer matrix.

But over the next thirty thousand years the brain overcame its insanity and took control of the starship’s entire computer system. Finally, he constructed a new, perfect human body for himself. After ten thousand years of mastery of the starship and the knowledge it gave him, the former exile, now styling himself the Master, was ready to embark on his mission of conquest.

However, he was unable to free himself from the starship complex, for he had become so integrated into the ship that it would not release him. Moreover, he determined that two of the Plodex who had been contained in the ship’s eggs had hatched and survived and existed at the present time: Marrina, now a member of the Canadian organization of adventurers called Alpha Flight, and the Plodex who was intended to be her mate, although she as yet knew nothing of this other Plodex’s existence, or even that she herself was an alien. The Master was determined to destroy both Marrina and her intended mate as vengeance upon the Plodex for the torture he had suffered. He broadcast a hypnotic signal to Marrina’s submerged savage Plodex instincts, forcing her to come to the starship. The starship’s mechanisms took her prisoner and tortured her. The other members of Alpha Flight came to her rescue and, as the Master had intended, destroyed so much of the starship’s mechanisms that the Master was freed from the star ship. When the starship exploded, the Master was hurled into the icy arctic waters below. He quickly swam to a large, fish-shaped submarine which he had begun constructing from the starship’s resources five thousand years before. The Master had hoped that Marrina too would have been destroyed, but she survived along with the rest of Alpha Flight and their allies on the mission, the Invisible Woman and Prince Namor the Sub-Mariner.

The Master found the monstrous killing creature that was the other Plodex and permitted it to commit a series of murders in Canada, knowing that Marrina would investigate the killings. She did, and as she approached the creature, she was greatly affected by chemicals it gave off, which triggered her Plodex killer instincts once more. She returned to Alpha Flight to attack her colleagues, at which time the Master captured Namor, Puck, and Marrina and took them to his submarine.

In the ensuing fight, Puck, by removing the Master’s helmet, unintentionally wreaked horrifying damage to the flesh on the Master’s head. Now, according to the Master, he could no longer control his vessel or the nonhuman Plodex, which began wrecking the submarine’s automatic systems in its fury. Namor, Puck, and Marrina, who shortly thereafter reverted to her normal human personality, all escaped. The submarine exploded, and the nonhuman Plodex was apparently destroyed. But Namor found no trace of the Master, and it is still unknown whether he is alive or dead.