Horus Heresy 2e Reactions

What?

Reactions are the new mechanic involved in the second edition of Horus Heresy.

How?

In the active player's turn the opponent can make moves or undertake actions as a consequence of what the active player does.

Huh?

It took me a few moments to truly "get it" as well. Here's how I thought about it. In older versions of the Warhammer 40,000 game there used to be such a thing as "over watch" (now appropriately called the same, but it is an official reaction in the 2nd edition of Horus Heresy). 

Once I'd figured that out, the rest of it is plain sailing! Reactions are just the same as facing an overwatch when I used to charge in with a unit. There's nothing more to it than that, and after a game or two, this is going to become second nature.

Beyond this, it is just a matter of counting how many reactions a player gets - usually 1 in each phase.

Evaluation.

In the movement phase, a unit can react if the enemy approaches closely. They can either go closer, or move further away. The choice between them depends on what the reacting player has in mind. Get closer to ensure that the charge from an enemy will succeed in some cases. Move further away to avoid a charge, or if you are playing Alpha Legion, to get out of enemy firing range (especially rapid fire range).

In the shooting phase, a unit can either return fire back at them (normal shooting), or can get shrouded at 5+ -- effectively damage mitigation. The choice between these should always be tilted toward returning fire when a unit is at a reasonably high survivor level, or probably use shrouded if there's not too many and if their weapons are relatively useless. Note the Wall of Death rule for flamers and the like here - no need to use templates at all.

In the assault phase, a unit can hold the line and force an enemy to be disordered in their charge which can certainly be very valuable when facing units who like to have bonuses on the charge, or they can overwatch right back at the charge -- again a powerful option. 

There are also Advanced Reactions (once per game antics) which can help in some situations. Interceptor and Death or Glory are now 2 of these advanced reactions and otherwise feel almost like they were in previous editions of the game. 

Overall then, reactions feel like a natural evolution of the game, and provide a smooth way to capture new dynamism that - in a minority of cases - has always existed within the game. Very happy with this development although as stated a bit further up, it will take a game or two to get used to.