You put a hex on a creature that temporary renders it incapable of recalling any memories.  The target is left unaware of what happened in the past and disoriented of their surroundings.  They don't remember how to use certain tools or weapons and they don't remember any skills.  For the duration of the spell, all skill proficiencies are reduced to 0, and ability checks rely solely on rolling a d20.  When the creature makes an attack, the attack roll bonus is ignored, so the value of the attack roll is determined only by the dice.  Any tool or armor proficiencies are also ignored, so apply disadvantages as needed.  To avoid the effects of Amnesia, the target must make a successful Wisdom Saving Throw.  The spell lasts for a maximum of 10 minutes.  Every 2 rounds, the creature can make a DC 17 Intelligence Saving Throw.  On a success, the spell is broken. 

At Higher Levels.  The spell's range increases by 10 ft. for each level increase above 4th.  The duration of the spell increases by 1 minute for every level.