Thorn bind hostage

When you cast this spell, choose a target you can see within range. The target must make a strength saving throw equal to your spell save DC. On a success the target breaks free of the spell and the spell wont trigger. On a failed save the target is retrained as 5 horned arcane vines rap around them. Until the target breaks free or the spell ends, the next five successful mele attacks on the target do an extra 1d6 pierceing damage. Once all five hits are made the spell ends and the target is no longer restrained.

Higher levels 

The damage goes up 1d6 for every slot above 2nd