Popular in the late 80s and 90s, Excalibur was originally Britain's own X-Men team including mutants from the X-Men with the addition of country's own heroes defending Britannia and the Otherworld. The most famous members are their leader Captain Britain with her sister Betsy, Rachel Summers, Nightcrawler, Kitty Pryde and Captain's beloved Meggan.
#20 A Wakandan wooden/elastic scientist who wants to become king. Excalibur and Black Panther enemy.
#19 Robot-like assassin who leads his own team of Janissary assassins. Acting under #1 while chasing his daughter Roma and Captain Britain.
#18 A female mutant mercenary with electromagnetic powers who chased Proteus' and Rachel Summers' DNA samples for Mr Sinister.
#17 An evil Nazi version of Excalibur, led by Hauptmann Englande from another universe.
#16 Evil inventor who wants to test the superpowers of others by making them fight each other in his Murderworld. Mostly Spider-Man and Captain Britain villain.
#15 The leader of his own "Mojo" world. A malevolent background character who has set his agents, especially #11 on the hunt for members of Excalibur.
#14 The power-hungry leader of R.C.X., who exploited the "Warpies" (Children from another dimension modified by #HM2) and their abilities.
#13 Nightmare-creating evil demon, used little nasty Bamfs (Nightcrawler-like creatures) to attack Excalibur.
#12 The evil necromancer who possessed Nightcrawler and attacked Excalibur to get the Soulsword for himself.
#11 Metallic wolves, minions of #15, who use the skins of their human victims to make their appearance. Excalibur's first and most common enemies.
#10 The evil X-Men group from another dimension, corrupted and controlled by #5 when he took over Xavier's mind.
#9 The leader of an inter-dimensional bounty hunter group "Technet", who became famous for serving #4.
#8 An evil AI created by Captain Britain's father James Braddock. Usually an enemy of Captain Britain (has also served as his JARVIS-like butler) who tried to take over the Otherworld.
#7 Sorcerer Supreme from another dimension, who got a portion of the Phoenix force by creating an opposite version of it.
#6 When it comes to the stories of King Arthur, of course he is one of the essential enemies too. The evil ancient witch who took over the mind of Captain Britain, making him her own black knight. The episode led to Brian losing his mantle to his sister.
#5 The psionic entity Excalibur has been encountered a few times, especially when he was leading his own group of X-men #10.
#4 Opal Luna Saturnyne's evil counterpart, former dictator of Earth-794 who escaped to Earth-616 by murdering Captain Britain's ex-girlfriend Courtney Ross and posing as her to plan world domination using #2. Later she has been the White Queen of the Hellfire club.
#3 Captain Britain's evil counterpart from another dimension, who was abandoned by #1 and began forming his own Captain Britain forces by killing Captain Britains from other dimensions and collecting their magic amulets and swords. The main enemy in the New Excalibur.
#2 Brian and Betsy's crooked brother, whose latent reality-warping powers were awakened by #4 for her own purposes, while he immersed himself in his own madness and began torturing Excalibur. He later became King of Avalon, breaking the Otherworld into several pieces and creating his own Captain Britains.
#HM1 A world-destroying cosmic being who confronted Excalibur to demand the removal of Rachel Summers and the Phoenix Force.
#HM2 An evil reality-warping creature and one of Captain Britain's main villains, who was imprisoned in the Otherworld where he, at the behest of #1, attacked Captain Britain troops and Roma using his Furies as aid.
#HM3 mutant Acolyte with the power to induce violence and hatred in others. He got Nate Grey to attack Excalibur.
#HM4 The Lord of Fear who tortured Excalibur members Nightcrawler and Rachel Summers.
#HM5 A strange group of villains from the early days of Excalibur. Originally the sidekicks of HM2.
#1 Excalibur is missing its archenemy but the closest I think is him. The old legendary wizard, the person who chose Captain Britain, originally benevolent but since his resurrection he has become increasingly evil and has started to play his own game, first by exploiting Excalibur and then attacking his daughter Roma, accusing her of a coup and eventually murdering her, and then by resurrecting King Arthur and attacking the entire Otherworld with his help to "cleanse" his own world of mutants he believes are polluting it.