chiron's guidence

Call upon the great teacher Chiron for guidance as his visage appears in the skies above. While concentrating on this spell you may add your spell attack modifier to attack and damage rolls. Every creature you hit while concentrating on this spell is marked. At the end of the spell's duration, the visage of Chiron shoots all marked targets dealing 1d8 radiant damage to each. you may use a bonus action to end the spell early.