Legends are rife with heroes who stood their ground against impossible odds - held back the crushing hordes and crashing waves - and prevailed. Dwarves especially revere this iconic heroic archetype, though other cultures often feature a similar gritty immovable hero. The Adamant Defender learns the secrets of making an alchemical treatment for iron or steel to temporarily give it the hardness of adamantine, and constantly improves the craft to forge a suit of armor custom built to exacting specifications. In tight confines where enemies cannot go around, the Adamantine Defender is an unyielding opponent. 

Adamantine Girding

At third level you gain proficiency with Smith's Tools and Alchemists Supplies if you did not have them already. With secret knowledge of how to make an adamant substance, once per long rest you can imbue one shield or suit of metallic armor with the Adamantine property (DMG p.150) which lasts for a number of days equal to your fighter level. 

Additionally, you have learned to stand your ground as a nigh-immovable object against what others might think are unstoppable forces. Whenever you are subject to forced movement or being knocked prone, you have advantage on your saving throw. If the effect does not allow a saving throw, you may make a Strength-based saving throw or ability check to resist the forced movement, moving only half the distance on a failure, or no movement on a success. 

Mastery of Deflection

At seventh level your skill with a shield exceeds others' simple proficiency. While making no movement and wielding a shield, you replace the normal shield bonus to Armor Class with your Proficiency Modifier. This effect applies if you are standing or riding on a creature, object or surface which is moving, so long as you make no movement of your own.

Juggernaught

Beginning at 10th level, while wearing adamantine-imbued or true adamantine armor you are better able to ignore damaging attacks, positioning your body and armor to avoid and deflect blows. If you are wearing medium or heavy adamantine armor, you may add your Constitution modifier to your Armor Class instead of your Dexterity modifier, ignoring any maximum-dexterity limitation.

Beat Back the Tide

Starting at fifteenth level, filling the breach has become your specialty, as you stand your ground and let your foes come to you. When you do not move on your turn (including while mounted or standing on a creature, object or surface which is moving) you gain advantage on all Strength saving throws, Strength-based ability checks and Strength-based attacks. 

Like a Glove

At eighteenth level, with years of practice and tailoring every individual section of armor to your exact specifications, your adamantine armor fits as if it were your skin. You may add both your Constitution and Dexterity modifiers to your armor class when wearing adamantine-imbued or true adamantine armor.