The Space Marines are the elite guardians of Humanity. They are the Angels of Death. They are the sons of the Primarchs who are sons of the Emperor and, as such, a part of his divinity.

In reality, they are genetically enhanced super soldiers created by the Emperor to fight in wars where regular men cannot fight.

Also, though there are over 1,000 Chapters of Space Marines in the Galaxy, each with over 1,000 Space Marines in them (Some more, some less), they are the knife-edge of Imperial war strength.

They are small and elite in comparison to the sprawling Imperial war-machine. Similarly, their forces on the battlefield are small and elite compared to most other armies.

Army Rules

Most armies have 1 Army rule that gives them a bonus. Becuase there re many sorts of Space Marine Chapters, they also have a rule to bring out the flavour of their army.

Space Marine Chapters

There are Epic Heroes in the game, characters that are named and have a significant story in the game. These Characters will have an additional Faction Keyword Associated to them.

All Space Marines will have "Adeptus Astartes" as a keyword. But Marneus Calgar will have "Ultramarines" as a second keyword and Vulcan He'stan will have "Salamanders" as a second keyword.

You cannot mix Chapters, ie second keywords. You build an army with Ultramarines or Salamanders, you have to choose and stick with one.

Additionally, there are more rules should you play a Black Templars, Space Wolves and Deathwatch army.

Black Templars may not use any Space Marine Datasheet with the Psyker keyword as they abhor psykers. Also they may not have the following units:
GLADIATOR LANCER
GLADIATOR REAPER
GLADIATOR VALIANT
IMPULSOR
REPULSOR
REPULSOR EXECUTIONER

This is because they have their own versions of these vehicles in their supplement Codex.

Space Wolves may not use any of the following units:
APOTHECARY
DEVASTATOR SQUAD
TACTICAL SQUAD

Deathwatch may not use any of the following units:
ASSAULT SQUAD
ASSAULT SQUAD WITH JUMP PACKS
ATTACK BIKE SQUAD
DEVASTATOR SQUAD
LAND SPEEDER STORM
RELIC TERMINATOR SQUAD
SCOUT BIKE SQUAD
SCOUT SQUAD
SCOUT SNIPER SQUAD
TACTICAL SQUAD
TERMINATOR ASSAULT SQUAD
TERMINATOR SQUAD

"Deathwatch" is a second keyword and thus cannot take units from other chapters.

Deathwatch may use the Killteam Cassius unit from the Agents of the Imperium Legends.

Now that that is out of the way, here is their army bonus:

Oath of Moment

In the Command Phase, select an enemy unit. Until the next Space Marine Command phase, any attack from the Adeptus Astartes units targeting this enemy unit can re-roll the Hit roll.

Additionally, Codex-compliant Chapters, (those who are NOT Space Wolves, Dark Angels, Blood Angels, Black Templars or Deathwatch), may also add 1 to their Wound rolls against that target.

For Example, if you play Imperial Fists or Ultramarines, you get re-roll to Hit and +1 to Wound. If you play Space Wolves or Dark Angels you get the re-roll to Hit only. Non-compliant Chapters have their own bonuses.

My Assessment

This is a very powerful ability that speaks to the single-minded conviction and determination of the Space Marines. Once they have declared an Oath of Moment target, they have determined that it must die.

Generally, players are increasing their chances of good damage output against a crucial target. I have seen people over-reach, using Oath of Moment (OOM) to increase their damage output in an unlikely situation and then fail to bring the target down.

It is also a waste to use OOM on a target that is 100% certain to die without its use. Example: One lone Cultist standing before a 10 man unit of Space Marine Terminators. I cannot even calculate the odds of a Cultist surviving a shooting or fighting attack from that unit. One in a trillion.

I feel that most of the time, when there are good targets on the board and you fail to destroy the OOM target, you had taken too large of a gamble and had not allocated enough resources to the destruction of that enemy.

An additional ability of OOM is targeting units that are inside Transports or in Reserves; this trick also applies to units that have been completely hidden from view. You wait for these units to show themselves in your opponent's turn and use the Overwatch Stratagem. Most units only Hit on a 6 using Overwatch, but you can re-roll the Hit rolls because it is your OOM target and it lasts until YOUR next phase.

This may significantly alter the way that your opponent plays, never mind the damage you might deal. They will become cautious and reserved. Or they may take a chance and tempt your best shooting unit to use Overwatch.

Similarly, you can target a unit you cannot see or shoot, but that will end in Engagement Rage (melee) of your units. Now, if your units survive, you can hit back and get re-rolls to Hit.

Conclusion

The Space Marines, Adeptus Astartes is a powerful army and have a powerful no nonsense Army Ability.

Support for the Hobby Fund

With my hobby posts, I generate a hobby fund. 50% of the income goes to @zakl.hobbyfund. What is interesting on that Hive account is my Grand Imperial army! The last post of which was the Warhammer 40k - Grand Imperial Army Post 23 - 2,855pts