At 1st level, a warlock gains the Otherworldly Patron feature. The following option is available to a warlock, in addition to the options offered in the Players Handbook.

The Shell

You have made your pact with a powerful being that possesses a shell resilient enough to block any strike. Your patron might be an ancient dragon turtle that has slumbered at the bottom of the ocean for centuries, a colossus formed out of the hardest metals and rocks making up the core of the planet, or another similar creature.

Alternatively, your patron's shell might not be a literal shell, but still a defensive part of them that they almost always have; they might be an automaton with a body made out of an unbreakable metal, or they could be a death knight that wears armor made out of the hardest steels known to man.

Expanded Spell List The Shell lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you. Spell Level Spells 1st heroism, shield 2nd aid, enlarge/reduce 3rd protection from energy, spirit guardians 4th death ward, fire shield 5th Bigby's hand, wall of stone

Shell Armor When you choose this patron, at 1st level, you gain the ability to form or retract a magical armor on your body, made from your patron's shell, as a bonus action. When your shell armor is formed, your AC is equal to 12 + your Constitution modifier. While you're wearing your shell armor, any critical hit against you becomes a normal hit. Additionally, your hit point maximum increases by 1 and increases by 1 again whenever you gain a level in this class.

Immovable Mountain Starting at 6th level, your patron grants you the ability to prevent yourself from being moved. You have advantage on ability checks and saving throws against being forcibly moved, and as an action, you can enter an immovable stance until the start of your next turn. While in your immovable stance, your weight increases by eight times, no creatures can pass through your space, and you cannot be moved by anything except gravity.

Defender's Body At 10th level, your skin reinforces itself with an iron-like sheen. At the start of each of your turns, you gain temporary hit points equal to half of your warlock level (rounded up). Also, when a creature within 30 feet of you takes damage, you can use your reaction to magically take that damage, instead of the creature. This feature doesn't transfer any other effects that might accompany the damage, and the damage can't be reduced in any way.