Resistance to psychic damage is already a goolock pact feature. Unlocks at 10th level. "Thought Shield; Starting at 10th level, your thoughts can't be read by telepathy, or other means, unless you allow it. Also, you gain resistance to psychic damage, and whenever a creature deals psychic damage to you, that creature takes the same amount of damage that you do." That's straight out of the 5e PHB. Fun fact, the last part of the ability makes a battle between two goolocks very interesting. It's not a may, so if two goolocks of 10th or higher fight, and one of them deals psychic damage to the other, thought shield will bounce the damage back and forth until one of them is dead. Not unconscious. Dead. As the ability does not specify that you must be conscious.