One game rule causes a mandatory action: If your Avatar is in the void, you may and must use their ability to play a site under them, ignoring normal site adjacency rules. Other effects may cause mandatory actions (e.g. Mask of Mayhem or The Green Knight). For mandatory attacks, the question of whether a unit 'can attack,' depends on their range of motion. You must resolve mandatory actions during your main phase before doing anything else. If you have multiple mandatory actions, do one of your choice, resolve it, and then continue doing mandatory actions one at a time until they are all resolved. To properly apply mandatory actions, simply ask: At this moment in my main phase, can I do the mandatory action? If yes, do it now! If no, do whatever you wish. If there is a mandatory action that you can't legally take, you can proceed with your main phase as you wish. If at any point during your main phase you become able to take a mandatory action, you must do so immediately, before continuing. Example - Avatar in the Void with No Sites in Hand Your Avatar is in the void, so the game rules require you to play a site under your Avatar, ignoring normal site adjacency rules. But, you don't have any sites in your hand, so you can't. Therefore, you are free to continue your main phase as you wish. You are not required to draw a site at the start of your turn, nor tap your Avatar to draw a site. But if at any point you have a site in hand and an untapped Avatar in the void, you must play a site under you! Example - Mandatory Attack Your Bosk Troll is 2 squares away from The Green Knight, The Green Knight is not within the Bosk Troll's range of motion, and thus you are not required to attack The Green Knight. You take actions in your main phase as you wish. Perhaps you cast a War Horse in the same square as the Bosk Troll. Then, you have the War Horse use the Pick Up basic ability, carrying the Bosk Troll. Then, you use the War Horse's Move and Attack basic ability, moving one square closer to The Green Knight. After resolving the War Horse's ability and the storyline is empty, the Bosk Troll is now able to attack The Green Knight. You must do so, in the middle of your main phase before doing anything else. You were not required to carry the Bosk Troll closer to The Green Knight. But once you chose to do that, the game noticed that The Green Knight was in the Bosk Troll's range of motion, and thus you were required to make the Troll's attack.