Boost is a token spell that can only be obtained from playing the Lunar Spellbook.
With the correct usage of Boost, your rival can take a substantial amount of unexpected damage from an enemy. Assuming the opponent will not kill his boosted enemy in once single hit, he will take damage equal to at least the enemy's attack + 5.
Boost's psychological effect is also very powerful. Some opponents will refuse to play their enemies during the chapter after a Lunar Spellbook has been cast in the hope that their rival's Boost or Group Heal will be wasted. This may however lead to a weak chapter and possibly losing the game.
It is recommended to use Boost as the first card of your chapter in order to maximize the chance of it buffing a rival's enemy.
Synergies and counters Edit
Boost can, in some situations, be stacked with Group Heal in order to massively buff your rival's enemy. This combo will usually result in the death of your unprepared rival.
The best possible play against a Boost user is to try to get them to waste it by having a support card only chapter.