Like all my 40k Tactics that I blog, it is important that you read the rule book yourself, I will make some basic assumptions on your knowledge, so please don't just try and do something and then quote this blog, make sure you understand it and can justify it with the rule book references.
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If you play with us on a Wednesday night down at the Battle Bunker, or have been around combat orientated players, you would have heard many people call for another person's opinion on whether or not someone could do a multi charge that they were trying to do. So I thought that it was about time that someone explained each of the rules that you need to follow in order to be able to create a multi combat, and I will start by just going over these points, then give examples on when you can or can't do it. All relevant material is on page 34 of the 40k rules, unless specified.
I am going to start this tactic with explaining the rules of moving assaulting models in detail, then show 2 examples of almost the exact same scenario, and why one failed to multi assault and the other succeeded.
First of all, who can you assault (this is page 33 of the rule book as well as the 3rd paragraph of "Assaulting Multiple Enemy Units" on page 34)? Well if you have not shot at anyone, then you can assault any unit as your primary unit (assuming you are in range and follow subsequent rules). If you have shot then the primary unit you charge must be the one you shot at (however after closest to closest you can then bring in other units if you follow the rules). So if you shoot at a unit, and then when it got to the assault phase either the unit had run away (and is no longer in range) or was wiped out, then you cannot assault. The exception to this rule is if you shoot a vehicle and the vehicle is killed - in this case you can assault the occupants of that vehicle (if any).
Once you have worked out who you can assault, you then must move closest to closest in a straight line. So if there is a piece of terrain that you could normally walk around but is in the way of the direct line between the closest to closest, then you must take a difficult terrain check. Once you have engaged the first model, the remaining models are moved in any sequence you like, with the following rules:
"Each model must end its assault move in coherency with another model in its own unit that has already moved."
This is the most import rule of consider. It is usually the first cause for someone failing a multi assault. So what this means is that obviously the 1st model has moved closest to closest. The second model, as long as they follow the subsequent rules, has to end its assault move within 2" of that one model. The third has to end its move within 2" of either the 1st or 2nd model, etc.
"If possible, the model must move into base contact with any enemy model within reach that is not already in base contact with an assaulting model."
So the first thing from this rule people don't understand is that this does not have to be the unit that you declared the assault against. This can be any enemy model, so therefore another unit nearby. You can only do this if you stay in coherency with models that have moved.
"If there are no such models in reach, the model must move into base contact with an enemy model that is already in base contact with an assaulting model."
So if following rules 1 and 2 you cannot reach a new model, that hasn't already been engaged, then you must try and engage one you already have if possible.
"If a model cannot reach any enemy models, it must try to move within 2" of one of its own unit's models that is already in base contact with an enemy."
This is another tricky one. So if there is a big enough gap between 2 units, this can stop you being able to assault because it is not stay within 2" of a model that has already moved (so not the same as rule 1), it is saying that you need to end 2" of a model that is in base contact. Some diagrams later will explain why this can make it difficult to charge.
"If this is impossible, it must simply stay in coherency."
This is pretty simple, if it can't engage an enemy model, and it can't get within 2" of one of your models who is already in base contact with an enemy, then it just has to stay with the unit. This is the rule that lets you bridge gaps between units and engage other units when the gap is large.
So onto the first example of why the player could not multi assault.
The Deathwing Terminator Squad has had its movement phase and is preparing to multi assault 2 X 5 man grey hunter squads that are 8" apart. The movement has been done sloppy and I will explain how they are unable to make the assault due to this movement.
The first move is always closest to closet. After that you can move the models in whatever order you like, but the gap is too big to cross while maintaining the 1st and 4th of the 5 dot point rules, Each model must end it's assault move within coherency of a model which has already moved and it must ends its move within 2" of a model already in base contact with an enemy model. As every single model can end within 2" of the 1st model that moved, the gap of 8" is too big to try and cross and the assault ends something like this.
Now lets look at the exact same scenario, but where the Deathwing player has moved the models in a manner that will allow the multi assault to occur.
At the end of the move the Deathwing player had conga lined them away from one of the units. Now when the assault moves are made the gap will be able to be filled.
Model 1 moves closest to closest, and model 2 gets into base contact with another model from the squad it is charging. Note: you don't have to do this move at this point, you have the choice in whatever order you want, you could go straight to the 3rd model and come back to the 2nd, you would still be following the rules, depending on what models are in what squads you may want to make sure pile in moves happen how you wish them to happen (eg. power fist, IC, etc where you want them). In this case it does not make a big difference.
Model 3 cannot make base contact with a model from either unit, and as a result just has to end 2" from either model 1 or 2 (as they have moved and are both in base contact with the enemy).
Model 4 cannot make contact with an enemy model from either unit without breaking the 1st dot point of keeping coherency with a model that has already moved, it also can't make it within 2" of model 1 or 2, so all it has to do is stay within 2" of model 3, so it can go further along the gap.
Model 5 can now make base contact with the 2nd grey hunter squad while maintaining coherency with model 4 (who has already moved) and as a result can now join the 3 units in combat.
Model 6 can't make base contact, but doesn't need to, as it can hit through other models anyways, and plus pile ins will move models into base contact with it anyways. Once all the models have moved, the assault will look something like this.
Then pile ins...
There are many people when assaulting multiple units do not follow all the rules, in the sequence they are dictated at, if you don't understand the 5 dot point rules, and you have a large gap between 2 enemy units, and you managed to pull off a multi assault, you were either an accidental ninja in the movement phase, or you did it wrong.
The art of the multi assault is in the movement phase, not the assault phase.