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According to the codex: "At the start of every Assault phase, the Ymgarl Genestealers must alter their form into one of those listed below. The benefits last until the end of the phase. Due to the instability of moulding their forms, they cannot choose the same alteration in two consecutive player turns"

Does this mean, if,

1) 5 Ymgarl charge into 10 Space Marines, strike first, pick +1 attack which equals to 20 attacks and 4 marines still standing. When the marines strike back, + 1 toughness to the Ymgarl in the same turn?

or

2) 5 Ymgarl charge into 10 Space Marines, strike first, pick +1 attack which equals to 20 attacks and 4 marines still standing. When the marines strike back, 2 Ymgarl still standing.
On the next player turn (the space marine side), the Ymgarl have to choose a different modifier so in this case it's + 1 toughness.

Which one is true?

(English is not my main so it's kinda confusing for me )

Number 2 is correct. The effect lasts until the end of a player's Assault Phase, meaning in practice that your Ymgarl Genestealers will swap powers every player turn, at the start of the Assault phase.

Xca|iber is right.

As above.

Assault Phase means the *entire* assault phase, not just the part where you attack

Correct, which all in all makes them pretty nasty but not as broken as in no 1