its a summon event that is added on a rotation with other events. So you want to save your scrolls and use it for the event. So that you can get the better rewords. You can see the upcoming event on the right side of the game where you do the weekly. You can see news and events

you want to save your summons for summon events so that you can collect the bonues rewords and you can get the coupon for the divine demona