Got a question:

I'm attacking with a unit with horrific mutation on it: "Attach to a target unit you control. While attached unit is attacking, defending units get -1 hit points."

The other player defends with his 1hp unit. So the opponent's unit is now 0hp, while I'm attacking. 

Question is:

Is this unit with 0hp automaticly destroyed while he is 0hp, or I must assign and apply him at least 1 damage token?

Rules says: "Hit Points: The amount of damage a unit can
take. If a unit has as many damage tokens as it has
hit points, that unit is destroyed and is placed in its
owner’s discard pile."

I would say, that a unit must have at least 1 token to be destroyed. But in my community there is a huge concern about that, so if you please try to answer.

Thanks a lot:)

I would say that during the Assign Damage step that the units that had been reduced to 0hp by the effect would be destroyed as they would have exactly as many damage tokens on them as they have hit points, ie zero. 

 I'm sorry, I meant the "Apply Damage" step.

Actually I believe the damage tokens equal to HP is not confined to the Apply damage Section of the Battlefield Phase, but a general rule. 0 HP and 0 tokens would mean death at any point, not just the apply damges phase.

"I am the Prince of Dorne. Men seek my favor."

As I read it, if you assign a defender with 1 hit point they die immediately. Page 7 of the rulebook says "If a unit has as many damage tokens as it has hit points...", and as this is not referring to any particular phase it would seem to apply regardless of timing.