Tortle Protector

You have mastered using your shell to better protect yourself and others. You gain the following benefits:

• Your natural armor trait now provides a base AC of 18. As normal, your Dexterity modifier doesn’t affect this number.
  
  
• As a reaction when you are forced to make a Dexterity saving throw, choose any number of creatures within 5 feet of you. Each chosen creature is considered to have half cover against any harmful attacks or effects while within 5 feet of you until the start of your next turn.
  
      The first use of this ability before a short rest comes at no cost (whether you save or not), each subsequent use where you fail the DEX save has a cost
  
  - 1st fail / short rest -> no extra cost
  - 2nd fail / short rest -> -1 AC until you take a long rest
  - 3rd fail / short rest -> -2 AC until you take a long rest (regain 1 AC per long rest)
  - 4th fail / short rest -> -2 AC until you take a long rest (regain 1 AC per long rest), and 1 level of exhaustion
  - 5th fail / short rest -> -3 AC until you take a long rest (regain 1 AC per long rest), and 1 level of exhaustion
  - 6th fail / short rest -> -3 AC until you take a long rest (regain 1 AC per long rest), and 2 levels of exhaustion
  - 7th fail / short rest -> -4 AC until you take a long rest (regain 1 AC per long rest), and 2 levels of exhaustion
  - 8th...