Way of the Unbreakable Warrior

Any Monk who has trained to become an Unbreakable Warrior is a fearsome sight anywhere on the battlefield, they aren't as calm or at peace as other Monks, and prefer to revel in the glory of battle. They hone their bodies to gain an unshakeable determination and pain tolerance. They practice brutal techniques and hone themselves to the fullest. Once this subclass is picked, the requirement of Dexterity and Wisdom is traded for Constitution and Strength. These monks now need 13 Con and 13 Str to multiclass in or out of being a Monk. 

LEVEL 3: Never Stop Fighting

If they are dropped to 0 Hit Points, a Monk in the Way of the Unbreakable Warrior can continue through their turns in combat as usual via sheer force of will, making a Death save at the end of each of their turns while continuing to make attacks. If they fail their death saves, they are dead as usual.

LEVEL 6: Improved Critical

The monks attack gets a critical hit on a 19 or 20. At level 11, this increases to be a critical hit on an 18, 19 or 20.

LEVEL 3: Unstoppable Blow

As an attack option, these monks can spend 2 Ki Points to use their Unstoppable Blow, the attack does bludgeoning damage equal to 2 rolls of their Martial Arts die, plus their Strength modifier. Both the Monk and target must both make constitution saves against the Monks Ki Save DC. The Monk makes it with advantage. If either fail, they take an additional 1d8 bludgeoning damage at the start of their turns until they are healed or take a rest. At level 17 and up, this damage increases to 3 rolls of their Martial Arts die.

LEVEL 11: Extreme Durability

The monk gains resistance to Slashing and Piercing damage, as well as immunity to Bludgeoning damage from non-magical attacks.

LEVEL 17: Art of Eight Limbs

The Monk can now attack four times per action, as well as their Flurry of Blows now granting 4 attacks instead of 2.

LEVEL 17: Unbreakable

The Monks who have achieved the Way of the Unbreakable Warrior now get an extra damage die when damaging those wearing armour, as well as they automatically succeed the saving throw for their Unstoppable Blow's effects. They may also now spend their Ki points to increase their Strength or Constitution scores by 1 for every 2 ki points spent, and this lasts for 1 minute. They also gain immunity to being stunned, paralysed or knocked unconscious.