Cthulhu's mind is overwhelming in its power and alien structure. He is immune to any effect that would sense his emotions or read his thoughts, and divination spells. This even foils wish spells and spells or effects of similar power used to affect his mind or gain information. The first time a creature other than a fiend or aberration makes mental contact with Cthulhu, it must succeed at a DC 27 Wisdom saving throw or contract indefinite madnessSRD. On a successful save, the creature cannot take reactions and can make either an action or a bonus action (not both) for 1 round; creatures that are immune to being stunned are unaffected. This effect can occur whether Cthulhu initiates mental contact (such as via a dream, or once per round merely by telepathic communication) or another creature attempts to do so (such as via detect thoughts or dominate monster). Once a creature is exposed to Cthulhu's overwhelming mind, it is immune to this effect for 24 hours.