If Kayn has moved twice (preventing him from taking damage) and has the Tank keyword, can attacking players ignore him and assign damage directly to other defending units?

Ruling: Yes, the attacking player can ignore Kayn and assign damage to other units. When a unit cannot be dealt damage, it is exempt from mandatory assignment considerations, which includes the Tank keyword requirement.

Sequence:

• Kayn with Tank who cannot take damage is identified
• Rule 443.1.d.9 applies: units that cannot be dealt damage are exempt from mandatory assignment
• The Tank keyword requirement is ignored for this unit
• Damage can be assigned to other defending units without assigning damage to Kayn first

Nuances:

• This only applies when Kayn cannot take damage (after moving twice); when he can take damage normally, Tank functions as expected
• The term "mandatory assignment" is only referenced in this one rule, making the interaction somewhat unclear in the rulebook
• No damage needs to be assigned to Kayn even though he has Tank, because zero damage cannot be considered "lethal" for a unit that cannot take damage