Teshar, Ancestor's Apostle

If the mana cost of a card in your graveyard includes {X}, X is considered to be 0.

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.

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 legendary land, as the Ixalan and Rivals of Ixalan double-faced cards do.