A Scroll is a powerful use-once spell which you can purchase and use in time of need. They are never found as random loot in the game - they can only be purchased from merchants or received as part of a quest's reward.
Their mechanics are similar to that of a Potion: various scrolls of the same type and quality will stack in a single 2x2 place in your inventory, for up to 5 scrolls. They can be assigned to the skill bar, too.
Scrolls vary in quality between the different difficulty levels, much like relics and charms. For instance, Scrolls in Epic mode are tinted blue and have the prefix "Major", whereas they're yellow-tinted and lack any prefix in Normal mode.