Displacement. Your brain, heart, and other vital organs are arranged in unusual positions within your body, causing Critical Hits to effect you differently. Rolling a nat 20 to attack you does not count as a critical hit against you, but rolling a 15 on the die does. If your AC is so high, a creature rolling a 15 on the die does not reach your armor class, it is not considered a critical hit, as the attack did not hit you.
Secret Weakness. A creature who is aware you have a displaced heart and brain, or one who witnesses you shrug off a critical hit, will typically attempt to deduce the location of your true weakpoints. They may make a DC 16 medicine check against you if they can see you but not touch you, or a DC 12 medicine check if they are able see and touch you. If they roll successfully, or are told the location of your displaced heart and brain, they can crit against you on a 15 and a 20. They are also no longer effected by your Special Circumstances.
Medical Anomally. Your biology in innately different than most. Surgeries become a minefield for accidentally hitting vital organs, and medicines often react differently in your body. All medicine checks made on you are at disadvantage. Medicine checks made on you which pass the DCs mentioned in Secret Weakness, immediately deduce that you have displaced organs, discovering your Secret Weakness.
Special Circumstances. Spells, skills, and magic items that trigger on a critical hit effect you differently.
A Battle Master Fighter's ability to crit on an 18, 19, or 20 has no effect on you, they still only crit against you on a 15.
Being decapitated on a nat 20 by a vorpal sword no longer instantly kills you, as your brain is no longer in your head.
You no longer take additional damage on a nat 20 from a Sword of Sharpness, but the secondary nat 20 still removes your limb, and retains it's DM described detrimental effect.
Adamantine armor does not grant immunity from critical hits. See "Displacement" for how to resist Critical Hits.
Any other spells, skills, and magic items that activate on a critical hit follow the same pattern. They do not deal additional damage, but secondary effects still apply, to the discretion of the DM.
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