356.2.a. Mandatory Additional Costs 356.2.a.1. Some Additional Costs specified by Passive Abilities on the card being played or another card are Mandatory, and must be paid to complete playing the card. They use the phrase "as an additional cost" and don't include the word "may." Example: A unit has the passive ability "As an additional cost to play me, kill a friendly unit." To play that unit, a player must kill a friendly unit. See rule 363. Passive Abilities for more information. 356.2.a.2. The cost imposed by the Deflect keyword is a Mandatory Additional Cost. See rule 809. Deflect for more information.