Armor of Bound Form (5e Equipment)
From D&D Wiki
Armor (Plate), Rare
This armor weighs 60 lbs instead of 55, as heavy bolts, extra plating, and reinforced bindings make it less comfortable to wear. Strangely, these additions don't seem to make the armor more resilient against outside attacks, but seem custom-built to keep the creature wearing it from acting freely.
Cast-Off. You can doff this armor as an action.
Immutable Form. Your physical form is magically and physically restrained while wearing this armor. You are immune to any spell or effect that would alter your form. This includes the altered forms of curses like Lycanthropy, though you still keep other effects such curses provide.
Curse. Some versions of this armor are baleful in nature, meant to punish shapechangers and Lycanthropes. If this is one of them, you can't willingly doff the armor, and are unwilling to be separated from it. Rather than outright preventing a transformation, a cursed version of this armor holds your form tight against it even as any shapechanging effect occurs, shrinking and warping around the new form incredibly painfully. Whenever your form is altered by a spell or effect, make a DC 20 Constitution saving throw. On a fail, you are wracked with horrendous pain that leaves you incapacitated until you revert to your true form. On a success, you must repeat the save at the start of each of your turns until you revert to your true form.