Una creatura attacca, quando durante la sottofase di Dichiarazione degli Attaccanti viene dichiarata attaccante (ossia la tappi, paghi gli eventuali costi per attaccare, etc etc). Una volta dichiarati tutti gli attaccanti, si innescano le eventuali abilità "Quando una creatura attacca" (come per il capitano). Dopo questo momento, non si innescheranno più abilità di questo tipo.
Eventuali altri creature che acquisiscono direttamente lo status di attaccante, infatti, non innescheranno questo tipo di abilità.
Comprehensive Rules ha scritto:
308.1. As the declare attackers step begins, the active player declares attackers. This game action doesn’t use the stack. [...]. Then any abilities that triggered on attackers being declared go on the stack. [...] Then the active player gets priority and players may play spells and abilities.
308.3. Abilities that trigger on a creature attacking trigger only at the point the creature is declared as an attacker. They will not trigger if a creature attacks and then that creature’s characteristics change to match the ability’s trigger condition.
308.4. If a creature is put into play attacking, its controller chooses which defending player or which planeswalker a defending player controls it’s attacking as it comes into play (unless the effect that put it into play specifies what it’s attacking). Such creatures are “attacking” but, for the purposes of trigger events and effects, they never “attacked.”
