"That was for appearances. Do I look like I need a bodyguard?"
Generally speaking, someone who has risen to the rank of Chapter Master, Master of Sanctity, or Chief Librarian is an all around badass who can take most of what the galaxy can throw at him. And in Warhammer 40,000, that's a lot. Honour Guards are less about protecting said badass (though should he run into trouble, they will naturally help him) and more about acknowledging a Space Marine's battle prowess and rewarding him by making him part of the Chapter Master's retinue. That said, Honour Guards will be called to serve in active combat, so it's a good thing that they can fight. Honour Guard are inducted from space marines with centuries of experience. Their wisdom is said to be greater than the Chapter's Captains, however they refrain from giving advice mostly, as otherwise it would usurp the recipient's authority. However, they do occasionally give out advice, and are respected to the point where it is considered folly for even a Chapter Master to ignore their words. Each Honour Guard has a Chapter Champion who trains in single combat to duel any challenger to his Chapter Master. Rather hilariously, the Ultramarines don't follow this particular edict of the codex astartes. Their Chapter Champion is not an Honor Guard, but a Captain (which is foolish when you think about it). Currently this position is held by Cato Sicarius.
Membership wise, the Honour Guard is overseen by the Captain of the Guard. Occasionally, the Captain will have a Guard Sergeant serve as his second in command, but often this post will be left vacant. The most senior members of the Honour Guard are the Chapter Ancient, who carries the Chapter Banner into battle, and the Chapter Champion, who is charged with protecting the Chapter Master.
Not to be confused with Honor Companies, which are more like seconded or pseudo-autonomous companies not restricted in number (number of honor companies, 100 Marines each) by the Codex Astartes. Chapters are said to heavily abuse this loophole. Given how Honor Guard promotion works, they probably aren’t limited in number by the Codex either...
On the Tabletop
Honour Guard have a respectable statline, being a space marine statline with an extra attack and ld10. The Chapter Champion is WS5 and A3, but must always issue or accept challenges if possible. They each come stock with Artificer Armour, a Bolt Pistol, a Boltgun, frag and krak grenades, and a power weapon. Considering that the Artificer armour and power weapon alone cost more than each honour guard, they are a bargain when it comes to point cost. Any member can also be armed with a relic blade, which is worth considering if your Chapter Master has the AP2 handled or if you have some extra points lying about. The Chapter Champion can also get a Thunder Hammer if he wishes (Remember, if you field him with a relic blade or TH, don't give him the free CCW! He won't benefit from it, so the boltgun is better. Only give him the CCW if you're fielding him stock with a power weapon). They are pretty much "light terminators" as they have a 2+ save and have a power weapon (compared to a 2+/5++ and a power fist). However, for some stupid reason they don't have relentless which means you can't fire your boltguns before charging (REMEMBER THEIR BOLT PISTOLS FOR THE LOVE OF THE EMPEROR). They are notably better than previous iterations as they are comparitively cheap and
actually worth taking now definitely worth taking now, if you have a chapter master and 110 points lying around, buy 4 to run around with him. Your enemy must now divert valuable AP2 firepower to deal with them, meaning they are extremely good when taken alongside terminators. Consider a drop pod for them as a cheap distraction unit with staying power. For some stupid fucking reason they can't take bikes so they can't be taken alongside smashfucker WHICH IS FUCKING BULLSHIT as if Chapter Master Smashfucker needed to be more broken as it is.
In 8th Edition they've been shifted to Warhammer Legends alongside the Legion of the Damned. It took a while for GW to give them a proper Primaris replacement, but better late than never: the Bladeguard Veterans fill their "awesomely detailed, greco-roman in space flavored bodyguards" niche.
Legion Honour Guard
Prior to the Horus Heresy, most of the Primarchs possessed an Honour Guard of some sort. Since Primarchs had the killing abilities of Chapter Masters but multiplied ten-fold, this was really more about rewarding a Marine's prowess than anything else. And even then. For instance, Lion El'Jonson had them go fuck shit up rather than stand at his side. Corax openly stated that his guards were only for show. Angron despised his so-called 'bodyguards', as did Konrad.
- Deathwing: The Deathwing started out as a specialized formation of Cataphractii Terminator-armoured elite, who carried out harrowing actions, shock assaults and strategic decapitation strikes. The unit charged to act as Honour Guards of the Lion, officers, and voted lieutenants was known as the Secta Mortis. They also served as body guards for Sanguinius after Konrad Curze murdered the Invictus and Sanguinary Guards. After the Horus Heresy, the Deathwing became the Dark Angel's (and their successor chapters) 1st Company and hunt down the
FallenHERETICS WHO ARE NOT RELATED TO THE EXTREMELY LOYAL DARK ANGELS.
- Phoenix Guard: The Honour Guard of Fulgrim. They wore Terminator Armour and carried ceremonial power spears. Even when the Emperor's Children fell to Slaanesh, they still served as Fulgrim's bodyguard.
- Iron Circle: The Honour Guard of Perturabo. Prior to the Horus Heresy Perturabo never used a bodyguard, but this changed when a squad of Imperial Fists made it to his personal chambers during the Battle of Phall. Instead of having legionaries staff these positions, however, Perturabo used Colossus-class robots from the Legio Cybernetica. They are armed with big hammers and shields.
- Keshig: The Honour Guard of Jaghatai Khan and other White Scars commanders. The Keshig was quite a diverse honour guard organisation as multiple sub-organisations existed within the Keshig, but the most common member was clad in Terminator armour. The Ebon Keshig were different terminator group wearing black terminator armour and was made up of legionaries sentenced to atone for dishonourable acts by protecting their charge, and the Golden Keshig were a group of Jetbike riding members of the organisation, who were presumably created when the Scars realised that terminators have trouble keeping up with bikes. Ultimately, their role was to keep anyone - other opponents and other White Scars - from getting involved and interfering when Jaghatai Khan was having some fun.
- Wolf Guard: The Honour Guard of Leman Russ. Within their legion Leman's guards were called The Companions and sometimes Thanes (because for once Black Library delivered the viking part of werewolf space vikings). When Russ left the Space Wolves, the only member of his Wolf Guard to be left behind was Bjorn. Even into the 41st millennium, Wolf Lords still use Wolf Guards to serve as their Honour Guards and senior sergeants.
- Templar Brethren: the Honour Guard of Rogal Dorn. They were the best swordsmen in the Imperial Fists Legion. Sigismund was their commander, as well as the First Captain. NOTE: Dorn's actual honour guard were called his Huscarls (which sounds more Space Wolf-y) and their foremost member was Archamus until he fell in battle against the XX Legion Primarch during the Horus Heresy. They are now reinstated by Guilliman to help the defense of Imperial worlds in the wake of the Indomitus Crusade.
- Atramentar: the First Company of the Night Lords and potentially Honour Guard of Konrad Curze. After the death of First Captain Jago "Sevatar" Sevatarion, the Atramentar scattered and protected errant Night Lord warband leaders of their choosing.
- Sanguinary Guard: the Honour Guard of the Primarch Sanguinius. The interesting thing is, the Sanguinary Guard took their job very seriously. Sadly, they were all killed with Sanguinius during the trip to the Vengeful Spirit, save for their founder Azkaellon. Following the Second Founding all Blood Angels successor chapters would maintain a number of Sanguinary Guard.
- Crimson Paladins: Part of Sanguinius' household, and although they didn't guard Sanguinius in battle (Terminators often find it hard to keep up with people that can fucking fly) they guarded places that he considered important, like specific chambers in the Blood Angels fleet.
- Morlocks: the Honour Guard of Ferrus Manus. They were all members of the Avernii Clan-Company, the First Company of the Iron Hands Legion. Most of them were killed at the Drop Site Massacre where Ferrus had his head lopped off by his erstwhile brother Fulgrim. By M41 Clan Avernii still exists as part of the Iron Hands Chapter, and they're still bitter fucks about what happened.
- Devourers: the Honour Guard of Angron. However, Angron had nothing but contempt for the Devourers, feeling bodyguards were pointless. By the time the Horus Heresy rolled around, they weren't even the best warriors in the World Eaters legion anymore.
- Invictus Guard: the Honour Guard of Roboute Guilliman. The name, literally "unconquerable", either harkens back to the Ultramarines' roots as SPESS Romans, or reflects vile fanboy influences.
- Deathshroud: the Honour Guard of Mortarion. They were Death Guard who were listed as killed in action and given Terminator Armour and Power Scythes to mimic Mortarion. They never strayed more than 49 (49 is seven squared and seven is Nurgle's favored number) paces from Mortarion. They still serve as Mortarion's Honour Guard even with Mortarion as a Daemon Prince of Nurgle in the later days of the 41st millennium. Sometimes he'll order a few of his Deathshroud to accompany a Death Guard commander to protect him/kill him if he fucks up his job.
- Sekhmet: the Honour Guard of Magnus the Red. Even when they wore Terminator Armour, Magnus was still twice their size. They were so emotionless, calm and disciplined, that hardly any of them noticed any changes when they all turned into dust-robots.
- Justaerin: the de facto Honour Guard of Horus Lupercal. While the Luna Wolves/Sons of Horus never had a formal Honour Guard, the Justaerin Terminator Squad was the premier squad of the already elite First Company, and often served in this role. They were commanded by Ezekyle Abaddon.
- Luperci: another of Horus' body-guard squads (who said you can't have multiples?), they were the Sons of Horus's first Possessed Marines, and unlike Justaerin they get to actually guard him in battle at least once. Against an entire Knight household, no less. Oh and they WON.
- Gal Vorbak: the Honour Guard of Lorgar Aurelian. They were the first Possessed Marines of the Word Bearers and were led by Argel Tal.
- Pyre Guard: the seven-man Honour Guard of Vulkan. They were drawn from the already elite Firedrakes and also served as his senior advisers. Loved fire even more than their Battle-Brothers.... Which is quite the feat. Also served as Chapter Masters within the Legion in addition to their Honour Guard role.
- Shadow Wardens: the Honour Guard of Corvus Corax. Corvus never felt the need for bodyguards but took them as everyone insisted. Following the Drop Site Massacre Corax dispersed his Honour Guard back into the companies, who desperately needed the manpower.
- Effrit Squad: the Honour Guard of Alpharius and Omegon. Essentially the Black Ops of the Black Ops Legion. Since the Alpha Legion kept Omegon's status as twin brother of Alpharius a secret, Omegon was passed off as head of Effrit Squad.
- Lernaeans: Another form of Honour Guard of the Alpha Legion. This time they were an elite formation of Terminators. Few knew of the Lernaean Terminator Squads. This was not because they sought to obscure their existence, but rather because they rarely left behind any witnesses of their deeds.
Here are all the legion Honour Guards in all their colors and flavors.