Quote Originally Posted by Fujin
Technically, thats an illegal move, but not for the reason you suspect.

"I forgot they could do that" lets you ignore an ability the monster has. Abilities are things listed in the back of the rulebook, such as Flying, Bloodlust, Leader, or Invulnerable. Support is not an ability, its a characteristic of the monster, same as its attack rating.

However, what would have been a legal play would have been to play "I forgot..." on Xykon's Horde ability, which would have immediately reduced the battle size to 5, (4th lair + Leader). Any monster beyond the 5th would be discarded, and Xykon's Attack and Defense would have been quite manageable for you :).

But yes, you can play "I forgot" during the "Play Monster Step", as its still within the combat section of the rules.

Hope that helps!
You seem to contradict yourself. Why do you consider the Horde ability (listed on page 31) to be valid for "I Forgot..." but not the Assist ability (listed on page 31)?