Surge

Surge is a talent which makes the player spend all their remaining resources in excess of the scroll's cost when casting it. The additional resources spent this way are referred to as the surge value in the scroll's effects, and it allows scrolls to be more powerful depending on that number.

This may be seen as an equivalent of variable costs found in other card games, except that players can't choose the amount.

In-game description
When this spell is cast, all remaining resources of its type are spent. The surge value is equal to the amount spent.