Your stone's endurance racial ability is improved in the following ways.
Stone's endurance's modifier is now strength or constitution, whichever is highest for you.
You can cast stone endurance twice per short rest.
You now gain temporary hit points due to an outer layer of hard rock encasing your skin after casting. The temporary hit points gained are equal to the number that was rolled with stone's endurance casting + modifier. The temporary health gained this way cannot stack with any form of temporary health other than itself, but the player can overwrite any other temporary health they have if they so choose. The temporary health gained this way is only applied AFTER the attack strikes you as the casing of rock needs time to form several layers.
You can choose to retaliate on any attacker within melee range, you are able to inflict a devastating headbutt and feel no pain when colliding with the enemy due to a concentration of dense cobblestone forming around your forehead. The damage for this attack is 1d8 + your strength modifier.
Due to the excess casting of this ability, you've also permanently increased the density of your skin at no extra cost to energy levels or stress on your body, increasing your AC by +1.