You don’t have to choose a target historic card to activate Abstergo Entertainment’s last ability. However, if you do, and that card is an illegal target at the time the ability tries to resolve, it won’t resolve and none of its effects will happen. No graveyards will be exiled.

A card, spell, or permanent is historic if it has the legendary supertype, the artifact card type, or the Saga subtype. Having two of those qualities doesn’t make an object more historic than another or provide an additional bonus—an object either is historic or it isn’t.

Some abilities trigger “whenever you cast a historic spell.” Such an ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

An ability that triggers “whenever you cast a historic spell” doesn’t trigger if a historic card is put onto the battlefield without being cast.

Lands are never cast, so abilities that trigger “whenever you cast a historic spell” won’t trigger if you play a legendary land. They also won’t trigger if a card on the battlefield transforms into a card with the legendary supertype, the artifact card type, or the Saga subtype.