Whenever you cast a spell of 1st level or higher during the duration, you can choose to echo it. To do so, expend a spell slot of the same level or higher than the echoed spell. If the spell requires attack rolls, you can reroll any number of those attack rolls, using the new rolls. If the spell forces creatures to make saving throws, you can force any number of creatures to reroll them, using the new results. A spell can be echoed only once per casting.