If a permanent or a card in a graveyard has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

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.

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.

If Havi and another legendary creature you control die at the same time, Havi's last ability will trigger for each of them.

In the rare case where a legendary creature becomes a copy of another legendary creature with greater mana value (for example, if Sakashima the Impostor is a copy of a legendary creature with mana value 5 or greater), then when that creature dies, if it's still a legendary creature card in your graveyard, Havi's last ability will be able to target it.

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.

Damage dealt to creatures remains on those creatures until the cleanup step or until an effect removes that damage. If Havi loses indestructible (probably because one or more historic cards in your graveyard left your graveyard) after having been dealt lethal damage earlier in the turn, it will die.

Use the mana value of the legendary creature that died as it last existed on the battlefield to determine what may be targeted with Havi's last ability.