The following was taken from the Warhammer Community page.
Let’s get down to the best phase – combat (unless you’re a Fire Warrior…sorry).
This is part of the new Warhammer 40,000 with some of the biggest changes. We’ve already seen in our article on unit profiles that Initiative has gone. Instead, the priority for striking is based on the previous phase, with those units that completed a charge swinging first.
There’s a definite emphasis on making charging into combat effective – these units have gotten all the way across the battlefield, they’ve braved enemy fire and overwatch, and now they’ve finally made it into combat – they will at the very least get to swing.
Units that activate gain a free 3″ move towards the closest enemy. This can be used to get within 1″ of other enemy units, if you’re cunning, dragging more foes into the melee and preventing them from shooting next turn, even if you didn’t charge them directly (giving them no chance to overwatch). Enemy gun lines will need to be careful about how they position their supporting units, so as to avoid getting dragged into the fight too.
Following chargers, players take it in turns to activate units across the board to fight – this can get quite tactical, as both players need to choose the combats where dealing maximum damage will be important to them, while trying to limit enemy retaliation on their valuable or fragile models.
There are a few units that can interrupt this sequence to attack out of turn too – Tyranids with lash whips and Slaaneshi Daemons, for example – and it can also be influenced by Stratagems (more on these soon) if your army is Battle-forged, all of which add a nuanced level of tactical depth to the phase.
Players will have much more influence over the outcome of combat now, rather than purely the stats of the models involved, both in their own and in the opponent’s turns (though we still wouldn’t expect Guardsmen to triumph over a unit of Khorne Berzerkers any time soon – fix bayonets!).
Another thing we have seen is that hit rolls are now fixed. This has the effect of making dedicated combat units generally hit on a 3+, while models representing the most competent warriors of the 41st Millennium (Guilliman, the Swarmlord, Ghazghkull Thraka, to name but a few) will now hit on 2+!
Close combat weapons (which we’ll look at in more detail in future) also gain new rules – some will slice through armour easily, while others will hit with enough force to cause deal multiple wounds that can cripple or kill even powerful enemy models.
Across the board, these changes lead to combats that are more deadly than ever. Generals who successfully coordinate a battlefield-wide charge will be rewarded with a phase of utter carnage, while their opponent will have to work hard to minimise taking damage, and carefully consider their retaliatory options.
Expect combats to be hard-fought, bloody, and tactical – just as they should be.