A paladin does not choose the oath of abolishment, but rather oath of abolishment choose them. From the point of their birth a rogue celestial (or not so much) power moved heaven and earth to arrange perfect circumstances for such paladin to be born. After all, this mysterious power was a powerful entity on a verge of death. Its last resort was to make a living vessel which could hold the last remains and memories of celestial entity.
A paladin, marked for such oath, had walked a difficult path their entire lives. Visions of unknown creatures and lands plagued them, whispers of something which might not be as holy as popular gods, and when other paladins get greater and greater magical powers from their devotion of a single cause, oath of abolishment paladin does not feel such connection to any existing deity... until one day their true destiny gets revealed.

Oath of abolishment is fueled by both vengeance and domination. Paladin of such oath get a small glimpse of loneliness and despair of a dying deity, and these paladins felt the taste of ambition of being the last one to serve this deity... of serving the last deity in the universe. Such paladin develops animosity to all other gods, especially those who receive lots of worship and praise, and this animosity drives them to restore the dead god back to is former glory... and beyond restoration - oath of abolishment means to abolish all other gods until only one remains. 

Forgotten Arts: Crusader's Siege

At 3rd level, while you are wearing heavy armor, you can use your Divine Smite feature with ranged weapon attacks and you gain +1 to attack rolls with ranged weapons.

In addition, while you are wearing heavy armor attack with a Shortbow, a Longbow or a Sling, then you can use your Strength for attack and damage roll instead of Dexterity.
While you are wearing heavy armor and attack with a Light Crossbow, a Heavy Crossbow or a Hand Crossbow, then your attack pierces your target and it can hit 1 addition target (roll to hit, but deal the same piercing damage as you dealt to your initial target) in a straight line (max range of 20 feet) behind your initial target.

Channel Divinity: Astral Siege

At 3rd level you learn how to channel your divinity to rip through walls of reality and unleash a torrent of astral energies on enemies. Target 10 -feet cube within 60 feet range of you, all creature within this cube will have to make a dexterity saving throw against your spell save DC. On a failed save, creatures will take (1d6 + your paladin's level) force damage and get their movement speed reduced to 0 until end of their next turn.

Twisted Fate of the Abolished One

At 7th level you learn how to twist fate to gain power over others.
As an action you can consume one 1st level spell slot to target an enemy and apply an effect on them identical to Hex spell, and apply an effect on yourself identical to Divine Favor spell. Both of these effects will last until end of your next turn and they do not require concentration to maintain. You can consume higher level spell slots to increase duration of these effects by 1 turn for each spell slot level higher than 1st.

Astral Siege Weaponry

At 11th level you can replace your Improved Divine Smite feature with this feature.
You learn Archery Fighting Style and your ranged weapon attacks now count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

Astral Void

At 15th level your Channel Divinity: Astral Siege feature become even more powerful. Instead of targeting 10-feet cube, you can now unleash this ability on all creatures of your choice within its range (you still need to see them).
In addition, you can choose 1 enemy who failed their Dexterity saving throw against your Astral Siege and summon a magical effect identical to Darkness spell at the location of the chosen enemy. This effect does not require concentration to maintain.

 

Avatar of the Abolished One

At 20th level you become the living vessel of your god and a lesser demi-god.
You gain these benefits:

• You no longer require sleep and magic can not put you to sleep;
• You reflect all charm and fear spells and effects back to their origin, treating them as if you used those spell or effects instead;
• You become resistant to radiant, necrotic and force damage;
• All enemies affected by your Channel Divinity become frightened of you until end of your next turn.