The Mirror

The Mirror subclass for warlock revolves around a manufactured demiplane that exists within reflections. This world was created as a secret prison to house a long forgotten deity thought to be dead even by others. After hundreds of centuries remaining inside this plane, this deity's power has been diminished, but is still a formidable force, and can grant power to those who seek to wield it that vastly differ from those given by a Celestial patron. The deity that this mirror-world and subclass revolve around is the thought to be dead deity Leira, The Lady of Deception, though there are many rumors and theories in the lore about this being her actual state, or if she faked her death to create her greatest illusion of all. It is up to you whether or not this plane is an actual prison for Leira, or if it is a realm of her own construction. How or why Leira ended up in this space is also unknown.

Reflect

Starting at 1st level, your patron bestows upon you the ability to cast a special form of the Shield spell. As a reaction, you can cast Reflect in response to an attacker attempting to break your armor class. Reflect will add +5 to your armor class, and if this reaction subsequently causes the attack to miss, the attacker takes half the damage they would've dealt, had their attack landed. When the Reflect effect is activated, an attack cannot be reversed back at a target again until the next time it is cast, and Reflect will remain as an ordinary shield spell until the end of the round. 

The amount of uses for this ability is dependent on your proficiency bonus. After your uses run out, you can't use the ability again until you finish a long rest.

Replicate

Starting at the 6th level, your connection to the mirror world allows you to mirror the spells of another creature for a brief moment. As a reaction, you can memorize a spell you have seen cast, regardless of class. On your next turn, you can choose to use your action to cast this spell. If you do not choose to cast this spell on this turn, you lose your memorization and can't cast the spell anymore. The spell memorized must be of a level you can cast, and using your action to cast it still expends a warlock spell slot. If you do not have any warlock spell slots to use, you can expend uses of reflect, and however many you expend equals the level of spell you are able to replicated (i.e. 1 use of reflect is a 1st level spell slot, 2 uses is a 2nd level, and so on). When casting said spells, use your own spell save DC and spell attack modifiers.

Once you use this feature, you can't use it again until you finish a long rest.

Man In The Mirror

Starting at the 10th level, you can summon a spectral, floating mirror as a bonus action at an unoccupied space you see within a 60 ft. range. Upon casting this ability, you immediately swap places with the mirror, in a manner similar to Misty Step. You can control this mirror as a bonus action on each subsequent turn, being able to move it 30 ft., or being able to swap with the mirror again,  but not both on the same turn.

The mirror has an AC and HP equivalent to that of casting Summon Shadowspawn at the 5th level, which would be an AC of 16 and an HP of 65. As an action, you can crash this mirror, summoning a replica of another creature you can see within range, that mechanically will use the stat block from Summon Shadowspawn. If the mirror has taken damage before summoning the reflection creature, whatever HP the mirror is at will transfer to the Shadowspawn (i.e. if the mirror is at 50 health instead of 65, the shadowspawn will be summoned at 50 health). Maintaining both the mirror and shadowspawn require concentration, and can only remain for 1 minute.

After using this ability, you can't use it again until you've finished a long rest.

Self-Reflection

Starting at the 14th level, you can summon a replica of yourself from the mirror plane as an action. This duplicate has half your health and half your spell slots, but is still able to cast the same spells as you, as well as has the same armor class. This duplicate also does not have access to the Self-Reflection, Replicate, or Man In The Mirror abilities, but does have half of your uses of Reflection. The duplicate gets it's own turn in combat and must assist you, but it can only perform actions and reactions, and the duplicate will always use Reflection as a reaction if it is about to be hit. Maintaining this duplicate requires concentration.

Once you use this feature, you can't use it again until you finish a long rest.