Bedrock Bulwark

When targeted with any attack that requires line-of-sight (an arrow, a dragon's breath attack, a spell like fireball, etc.) you drop to your hands and knees and channel your will through the Elemental Gemstone. You pull a rock out of the ground directly in front of you, deflecting the attack and permanently changing the terrain. If this spell cannot be cast in the exact space as a structure, but it may be cast next to one. The rock is 20 feet long, 10 feet high, and 5 feet thick. The rock has an AC of 15 and 30hp per inch of thickness and is immune to poison and psychic damage. This rock counts as full cover for you and any creatures for the purpose of the triggering attack. If the attack would be blocked by the attack, the rock takes the damage. If the attack specifies the ability to move around walls it can damage the rock and still damage the caster (such as a spell like cloudkill). Any creatures in the area of the rock are lifted on top of it unless they wish to make a Dexterity Saving Throw to attempt to roll off the rock. On a success they may choose which side of the rock they land on. Normal climbing rules apply.

This spell can also be cast as an action, without needing to be targeted by an attack.