If a card with escape is in your graveyard, you can cast it. Escape provides you with an alternative cost with two components: an amount of mana you'll have to pay and a number of other cards you'll have to exile from your graveyard. Escape doesn't change when you can cast the spell. Instants with escape are good to go anytime; the other card types can be cast only during your main phase when the stack is empty.

As an ability word, constellation doesn't have a specific rules meaning. It's used to shine some starlight on abilities that trigger whenever an enchantment enters the battlefield under your control. Each constellation bonus is different.

Several cards in the Theros Beyond Death set encourage you to be clever and cast spells during your opponent's turn, giving you a reward the first time you do so.

The pantheon of Theros is mighty, and the very presence of its members will change your game. But if you show enough devotion to the gods of Theros, they'll take a more active hand in fighting beside you.