HMS Eagle was a British Invincible-class VTOL aircraft carrier seen in the Hellsing manga and the Hellsing: Ultimate OVA series. It was hijacked by Millennium forces as part of their grand plan to invade London.
Sometime before Millennium's planned assault on London, three members of the Eagle's crew, including at least one officer, agreed to help Millennium in their plans in return for becoming vampires. Several coffins carrying Millennium soldiers were hidden in the cargo hold as well.
After Schrödinger delivered the Major's declaration of war on England to the meeting of the Convention of Twelve, Iscariot, and the Queen, Millennium First Lieutenant Rip van Winkle took a helicopter out to sea, heading for the Eagle. Her singing was the cue for the vampires on board to attack the rest of the crew, turning them into ghouls and leaving the ship in the hands of the Nazis. The three vampiric collaborators emerged on the deck to greet Rip, only to be killed by her magic bullets. As her fellow Millennium soldiers awoke and emerged from the cargo hold, Rip used blood and paint to decorate the carrier's deck with an enormous Nazi swastika. Redubbing the ship the "Adler", Rip van Winkle had her fellow vampires man the controls while she sat on the deck and waited for the moment when Millennium would begin their attack on London.
Under the direction of Sir Shelby M. Penwood, British forces attempted to retake the Eagle with a squadron of SAS helicopters, only for the entire force to be shot down by Rip. A later attempt to destroy the carrier with fighter jets met with similar failure, prompting Integra to realize that they needed some means of approaching the ship undetected in order to deploy Alucard in combat range of the enemy vampires. To this end, Walter suggested the use of a SR-71 Blackbird, whose flight capabilities would allow it to approach the Eagle undetected.
Alucard proceeded to use the jet as both transport and weapon, crashing it into the deck of the Eagle and setting much of the carrier ablaze. Although the powerful vampire killed all of the Millennium forces aboard the ship, the Major revealed that this was all part of his plan; with his plane destroyed, Alucard had effectively stranded himself at sea, allowing Millennium to attack London without his interference. To circumvent this, however, Alucard used his powers to take control of the ship itself, slowly sailing back towards England. Once the ghost ship reached the Thames River, Alucard was able to leap into the city and rejoin the battle.