You create magical ruby armour to appear on your body. The armour will have jagged and sharp edges and spikes. Despite the appearance the ruby armour is comfortable on you, will accommodate your clothes, does not hinder your movement, and will reshape itself to ensure it does not interfere with actions like sitting, leaning on walls, and will not damage anything except anyone that attacks you.

This ruby armour grants a magical bonus to your Armor Class equal to your proficiency bonus. Any creature that hits you with a melee attack or successfully grapples you receives piercing damage equal to your proficiency bonus. 

While this spell is in effect it will interfere with some other protective magics: you cannot benefit from any other magical bonus to Armor Class from any other spell of 4th level or lower or any magic item of Common, Uncommon or Rare level of rarity. This does not affect anything that grants an Armor Class calculation, however, so this spell can be used with spells like  Mage Armor but cannot be used with spells like Shield of Faith or Shield.