Brambleskin

You cause a creature you touch to grow thorny bramble growths on their skin and clothes. 

For the duration of the spell, whenever you take damage from a melee attack, the foe attacking takes 1d4+spellcasting modifier piercing damage as brambles shoot away from you at the enemy. If the enemy's attack is a critical hit then roll 2d4+spellcasting modifer piercing damage from the explosion of bramble thorns.