You gain oath spells at the paladin levels listed.
Oathbreaker Spells, Paladin Level
3rd: Hellish Rebuke, Inflict Wounds
5th: Crown of Madness, Darkness
9th: Animate Dead, Bestow Curse
17th: Contagion, Dominate Person
When you take this oath at 3rd level, you gain the following two Channel Divinity options.
Control Undead. As an action, you target one undead creature you can see within 30 feet of you. The target must make a Wisdom saving throw. On a failed save, the target must obey your commands for the next 24 hours, or until you use this Channel Divinity option again. An undead whose challenge rating is equal to or greater than your paladin level is immune to this effect.
Dreadful Aspect. As an action, you channel the darkest emotions and focus them into a burst of magical menace. Each creature of your choice within 30 feet of you must make a Wisdom saving throw if it can see you. On a failed save, the target is frightened of you for 1 minute. If a creature frightened by this effect ends its turn more than 30 feet away from you, it can attempt another Wisdom saving throw to end the effect on it.
Starting at 7th level you, as well any fiends and undead within 10 feet of you, gain a bonus to melee weapon damage rolls equal to your Charisma modifier (minimum of +1). A creature can benefit from this feature from only one paladin at a time.
At 18th level, the range of this aura increases to 30 feet.
At 15th level, you gain resistance to bludgeoning, piercing, and slashing damage from nonmagical weapons.
At 20th level, you can, as an action, surround yourself with an aura of gloom that lasts for 1 minute. The aura reduces any bright light in a 30-foot radius around you to dim light. Whenever an enemy that is frightened by you starts its turn in the aura, it takes 4d10 psychic damage. Additionally, you and any creatures of your choosing in the aura are draped in deeper shadow. Creatures that rely on sight have disadvantage on attack rolls against creatures draped in this shadow.
While the aura lasts, you can use a bonus action on your turn to cause the shadows in the aura to attack one creature. Make a melee spell attack against the target. If the attack hits, the target takes necrotic damage equal to 3d10 + your Charisma modifier.
After activating the aura, you can't do so again until you finish a long rest.
Aura of Malevolence
Prerequisite: 21st level
Your Aura of Hate extends its reach. The range of your Aura of Hate increases to 20 feet. Additionally, any creature within this aura that is hostile to you has disadvantage on saving throws against being frightened.
Desecration
Prerequisite: 21st level
You can corrupt the ground beneath you with your malevolent energy. As an action, you can desecrate a 20-foot radius centered on you. This area becomes difficult terrain for creatures other than you and your allies, and any healing effects within this area are halved. This effect lasts for 1 minute.
Shadow Infusion
Prerequisite: 23rd level
You can empower your attacks with the darkness that fuels your soul. When you hit a creature with a melee weapon attack, you can choose to infuse the attack with shadow energy. The target must succeed on a Constitution saving throw (DC = 8 + your proficiency bonus + your Charisma modifier) or be blinded until the end of your next turn. You can use this ability a number of times equal to your Charisma modifier (minimum of once) per long rest.
Soul Harvester
Prerequisite: 25th level
Whenever you reduce a creature to 0 hit points with a melee weapon attack, you can choose to consume its essence. You regain hit points equal to the damage dealt by the attack, and the creature's soul is trapped in your weapon for 24 hours. While a soul is trapped in your weapon, you gain a +2 bonus to attack and damage rolls made with that weapon.
Unholy Resilience
Prerequisite: 27th level
Your bond with dark powers grants you unnatural resilience. You gain resistance to radiant damage, and you have advantage on saving throws against being turned or banished. Additionally, when you are reduced to 0 hit points but not killed outright, you can choose to drop to 1 hit point instead. You can use this feature once per long rest.
Lichlord's Ascension
Prerequisite: 30th level
You ascend to the pinnacle of unholy power, becoming a being feared by both the living and the dead. You gain immunity to radiant damage and the frightened condition. Additionally, when you are killed, you automatically return to life after 1 minute with half your maximum hit points, unless your body is completely destroyed. Upon returning to life, you can choose to create a wave of necrotic energy in a 60-foot radius, dealing 8d8 necrotic damage to all enemies in the area. Half the damage dealt by this wave is returned to you as hit points.