Capabilities - I feel the key to understanding where and when to apply a unit in any given situation is informed by knowing their full capabilities off by heart; here is a look at what this unit is capable of.
The Chapter Master of the Ultramarines is an exemplar of his own splinter of the peerless thirteenth legion, contributing to a match in nearly any capacity you can think of; he is relentless in his drive to survive any blow, he is nearly unmatched in close quarters engagements and he adds a wealth of strategic bonuses to whatever force he elects to lead into battle. He is living proof that the standard Chapter Master profile can give way to a master of the battlefield, being highly skilled at both melee and ranged fighting with his Weapon Skill 6 and Ballistic Skill 5, while he is suitably fast with Initiative 5 as well as being virtually immune to Leadership effects given that his titular value is ten and he possesses the And They Shall Know No Fear special rule. His average Strength 4 and Toughness 4 are heavily augmented by his wargear and abilities, while 4 Wounds and 4 Attacks ensure that he can both withstand and reciprocate any blow. He can be attached to any eligible unit due to his Independent Character standing and can benefit from all of the Combat Doctrines one at a time in each encounter, improving upon his already considerable damage output at any distance. Speaking of range, Marneus' wields a specialized set of bolt weapons that combine to fire two Strength 4 AP2 shots with a range of 24" and using the Assault type, being highly precise and decently powerful weapons that can easily fell a Terminator or other elite model from afar while still allowing Calgar to declare a charge in the same turn. To add to this, the Lord of Ultramar is also equipped with an Orbital Strike to perform a powerful one-use-only ranged attack that can only be used if he does not move (if he is not armed with the Armor of Antilochus) but doesn't prevent him from charging the target unit in the same turn contrary to most Ordnance weapons. This attack manifests as a single shot that is a Strength 10 AP1 Large Blast with the Ordnance (roll 2D6 and choose the highest for armor penetration) and Barrage (wound allocation is resolved from the centre of the blast marker, ignores directional cover, can be fired without requiring line of sight) special rules. While it is incredibly powerful given its profile, the fact that it can be fired at any unit with its infinite range and lack of necessary line of sight, it is also incredibly inaccurate as it can never use Calgar's Ballistic Skill to reduce any applicable scatter.
Where Marneus has truly made a name for himself on the battlefields of the Warhammer 40,000 table-top game is in the depths of an assault, using his raw might and durability to outlast nearly any foe with an alarming success rate. Though he is armed with a power sword, this is but a token weapon that functionally serves no use in the game other than to try and stay in a combat longer, while his krak grenades are also of minimal value; the frag grenades, on the other hand, ensure that Calgar strikes at Initiative on any turn in which he charges into cover. Indeed, it is in his specialized pair of power fists aptly named the Gauntlets of Ultramar that Marneus places his faith; these function as two separate power fists and thus provide the Chapter Master with a bonus attack while also doubling his Strength 4 to Strength 8 and adding AP2 to the mix. The Gauntlets of Ultramar are thus capable of reliably tearing apart anything short of a Toughness 10 or AV14 model while punching through any type of armor and become especially deadly when you consider the Weapon Skill 6 model with five base attacks (or six on the charge) that wields them. However, their true value shines forth with the jaw-dropping omission of the Unwieldy special rule, signifying that all of those attacks are resolved at Marneus' Initiative 5 step. If that doesn't immediately fill you with either awe or dread, let me illustrate what a cataclysmic difference that one rule makes; most AP2 weapons in the game are Unwieldy, feature on Initiative 4 and lower models such as those using the Smash rule (monstrous creatures mostly). Marneus Calgar has a 2+ save and can thus withstand almost any attack that comes at or before his Initiative step, while he can beat most enemies using AP2 weapons to a pulp before they get a chance to strike. This not only ensures that Calgar will survive longer in a match but also has the knock-on effect of allowing him to fight in more exponentially more assault phases, ensuring he does even more damage to the enemy army until they are forced into submission. There aren't many characters in the game that can match Calgar for raw at Initiative death dealing given that he can similarly punch walkers such as Dreadnoughts to oblivion before they can retaliate, while even Gargantuan Creatures such as the mighty Wraithknight aren't safe from his blows.
Of course, no combat beat-stick would be complete without the resilience to withstand counter blows in equal measure given the competition they face, and Calgar is no exception to that rule. While Toughness 4 lets most attacks wound him rather easily, his 4 Wounds are a major turn off for foes looking to kill him quickly; this is due in no small part to the fact that he is an Eternal Warrior, guaranteeing that your opponent must put a huge investment into taking off each individual wound just to kill a single Space Marine. His 2+ armor save and 4+ invulnerable save stop just short of being the best saving throws a character can reasonably expect to have, but even the latter can be improved courtesy of psychic powers such as Sanctuary. On the subject of armor, while Calgar is equipped with a suit of artificer armor as standard, he can optionally replace it and both the frag and krak grenades with the Armor of Antilochus, a unique panoply of war derived from those worn by Terminators. While losing out on assault grenades is a nasty loss to suffer and the defensive stats are unchanged regardless of what armor suit you use, this option is nonetheless an interesting one even despite being a paid-for upgrade; nominally, it confers the Relentless special rule on Calgar to allow him to move and fire his Orbital Strike in the same turn. Otherwise, it includes a Teleport Homer to aid Deep Striking Terminator squads and runs opposite of regular Terminator frames by not preventing him from performing Sweeping Advances, a very useful bonus indeed.
While it is undeniable that Marneus Calgar is not quite the general of a battlefield as he was in previous codices, he still provides more support abilities than most characters in the codex and this is all to your benefit. The first of these is that Marneus can choose any single Warlord Trait you wish for him to use from the Space Marine Warlord Traits table, enabling you to select the most optimal trait for use in any particular match-up. Being able to guarantee the provision of the incredible Storm of Fire trait and its associated Rending bonus to any friendly ranged unit within 12" or confer the Feel No Pain (5+) special rule on himself are the two most obvious uses of this ability, but you can reasonably expect to get mileage out of most of the traits depending on the circumstances. It is important to note that the wording of the rule doesn't seem to suggest that he is forced to use the Space Marine Warlord Traits, but whether this is the case or not should be discussed with any applicable gaming partners as having the option to instead roll on another chart is always useful. Lastly, Calgar embodies the role of a Master Tactician and thus enables your Ultramarines forces to benefit from any single Combat Doctrine twice in the same game, essentially allowing you an extra turn of guaranteed re-rolls on failed to-hit rolls for one or both of shooting and melee attacks. While it is difficult to properly utilize alongside a Gladius Strike Force and the Battle Company, any other army build gains considerable advantages from it considering how impressively it buffs the average damage dealt by Space Marines that are traditionally known as being a strong but not over-powering ranged force (outside of certain offenders such as Centurion Devastators, of course). It is but one of many impressive traits possessed by the Chapter Master of the Ultramarines and is a fitting one to close on.
Where to Place Them - Positioning is a crucial element of any match, something that is especially true in the adaptive gameplay of 7th Edition; this is my recommendation on placement for this unit.
I would recommend using Calgar in a similar vein as Lysander, that of an incredibly tough wound tank that ignores Instant Death attacks and soaks up a typhoon of punishment - this is especially the case if you choose to give him Feel No Pain (5+) from a certain Warlord Trait. He can eat the wounds his bodyguard simply cannot afford to die for based on anything from not wanting to lose melee or ranged efficiency to keeping Objective Secured models alive to steal a valuable victory point. However, don't get the crazy idea that he can be sent off on his own as Calgar is still fundamentally four Terminators with Iron Halos as opposed to Storm Shields in terms of surviving wounds; you want a good unit to keep him safe from the attacks that will kill him such as those generated by Destroyer weapons. Honor Guard are the natural choice given that they share a kit with him, are arguably the best overall close combat unit in the codex and don't suffer from the many limitations that Terminators have to endure, though their lack of invulnerable saves also gives you reason to consider investing in the more expensive but much tougher Assault Terminators armed with thunder hammers and storm shields. Even Assault Centurions line up nicely with the Lord of Macragge given that they too feature at-Initiative AP2 attacks as one of their major selling points, but like with the Honor Guard, they don't anything that can save them against AP2 weaponry short of cover or hiding behind Marneus' over-sized fists. Whatever you choose, know also that Calgar is right in the top echelon of melee beat-sticks and as such can actually justify the use of a Land Raider variant, a psyker using the risky but much cheaper alternative in the Gate of Infinity, or you could even keep it on the cheap side and use something like a Drop Pod. Keep in mind that unless he is equipped with the Armor of Antilochus, he can also ride along in a Rhino, Razorback and other "small" transport to suit your needs.
Best Uses - It is true that each unit in the game ultimately has one or a handful of tasks they are best suited for in a general sense; these are the most efficient uses of this unit based on their capabilities.
I'm sure you've probably read or even heard this before, but it really should be repeated; beat-sticks like to be leading the charge and getting into combat as quickly as possible so as to not waste the fact that they mostly contribute to the game by killing things at melee range. While this thankfully isn't fully the case with Marneus Calgar, it's still sensible in this case given that his greatest asset is his ridiculous power in an assault. Unlike most other characters in the codex, there's actually very little that Marneus can't justifiably pummel and thus be sent head-long into like a bullet given that even the undisputed queen of challenges - Jain Zar - will get destroyed by his pair of melee weapons, rendering her disarming ability worthless against him (his power sword ensures that he still gets a bonus attack). He also serves as a natural counter to dedicated assault units given that he is one of few characters in the game to provide a mess of incredibly high Strength AP2 attacks that strike at a an above average Initiative step with a good Weapon Skill value; he can readily cut through Thunderwolves or Flesh Hounds and fail to break a sweat while doing so, giving you an alternative to simply avoiding or shooting said nasty assault squad down. As far as determining which Warlord Trait to select is concerned, if you don't expect Calgar to be lining up near a potent ranged unit, you can afford to skip Storm of Fire, but even with that said it still ranks as easily the best Warlord Trait in the codex. The Champion of Humanity trait is ideal if you utilize Allies from other Armies of the Imperium forces, while the Iron Resolve trait is good if you want to keep him alive as long as possible. Which Combat Doctrine you choose to re-use is also heavily dependent on the match-up and the turn-by-turn circumstances as you could very well end up needing any one of the three more than another at a given point, though you could say that getting to grips with this is part and parcel of being an Ultramarines player given that they are the only Chapter that always uses the Combat Doctrines.
Thank you all for reading this article, I hope you enjoyed it! While Marneus has steadily transitioned from a generalist power-house to more of an ultimate warrior and king of the challenge, I still feel that he is an immensely useful character for any Space Marine army and his position as a Lord of War doesn't significantly clash with his innate value. He is no doubt one of the strongest infantry models in the game short of a Primarch and I am honored to have him feature as the final review of my Space Marine Tactica series. With that in mind, I must implore you to leave me any feedback you have both for this and any previous authoring I've done for Space Marines; time has been my enemy lately and this has led to me dealing over-long with both the Adeptus Astartes and the Orks (especially the latter) despite a slew of new armies and codices awaiting my reviewers' touch. I'm excited to start fresh with the books released in 2015 and I'm always thankful for your continued support. Have a lovely day and enjoy your war-games!
We follow in the footsteps of Guilliman. As it is written in the Codex, so shall it be.
- Marneus Calgar, Chapter Master of the Ultramarines