Cataphract

The Cataphract is, at its core, a super-heavy cavalryman, the medieval version of a main battle tank. Capable of shock attack and extended fighting, the cataphract is a mounted juggernaut, both horse and rider clad in the heaviest armor possible. 

Mighty Steed

When you first take this subclass, you gain access to a Large Beast of CR 1 or less. Whenever you gain a level, the steed gains a hit die, and its proficiency bonus increases when yours does. If this beast or you wish to change it, you must spend 8 hours acclimating the steed to you. A new steed's CR must be equal to or less than half your level, and must be a Beast.

Born to the Saddle

Starting at 3rd level, your mastery as a rider becomes apparent. You have advantage on saving throws made to avoid falling off your mount. If you fall off your mount and descend no more than 10 feet, you can land on your feet if you’re not incapacitated.

Finally, mounting or dismounting a creature costs you only 5 feet of movement, rather than half your speed

Cataphract's Charge

Also at 3rd level, you can exploit the ultimate heavy cavalry attack: the charge. Once per turn, if you move at least 20 ft. directly toward a creature and hit it with a melee weapon attack, you deal an extra 2d6 damage of the weapons type and can force the target of the attack to make a Strength saving throw. If the target fails, the shock of the impact knocks the target prone.

Companions on Campaign

At 7th level, you support your companions, humanoid and animal, on long and lonely marches. You and other creatures traveling with you have advantage on saving throws against exhaustion imposed by a forced march, and you and your mount alike no longer suffer movement penalties for wearing heavy armor. Further, when you make the attack action, you can forgo one of your attacks to allow your mount to make an attack. 

Armored Juggernaut

At 10th level, as long as you are wearing heavy armor, you gain resistance to all damage, except Radiant, Necrotic, and Psychic. 

Line Breaker

At 15th level, any creature you damage during your turn cannot take opportunity attacks against you or your mount until the end of your turn, allowing you to position yourself for powerful charges as you see fit.

Ultimate Steed

Beginning at 18th level, you can gain a Monstrosity as your steed. Doing so requires one week with the Monstrosity, and you must spend at least 6 hours a day with it. At the end of that time, it gains Hit Dice equal to the difference between your level and its current Hit Dice (if its HD are lower). Further, you can forgo one of your attack in an Attack action to allow your mount to make one of its listed Attacks.