When you realize a villain is trying to target (via attack/spell) someone or something besides you within range of this spell, you may cast Taunt as a reaction. Make an opposed check of your spellcasting ability vs their Wisdom.  If you win the check, then they redirect the attack to you.  Otherwise Taunt fails entirely and resolve the attack as normal.

 

If you are not a valid target of the attack/spell, then the attack fails (i.e. wizards can not have continual flame cast on them).  Otherwise resolve the attack as if it targeted you normally.

 

Note, Taunt will work on target-able area effect spells, but the original target may still be in the area of effect.  That is, shifting a fireball from targeting a space 5 ft from you to center on you is rarely helpful.