Divine Vessel

You gain the Magic Initiate feat from the Player's Handbook and must choose cleric for the feat.

Channel Divinity: (Preserve Life or Take Life)
As an action, you hold out your hand and evoke either your life or death energy. This energy either restores or damages a number of hit points equal to five times your level. Choose any creatures within 30 feet of you, and divide those hit points among them. This feature can restore or harm a creature to no more than half of it's hit point maximum. You can't use this feature on an undead or a construct. You may use this feature once per long rest. After a long rest it replenishes.

Lay on Hands: (Blessed Touch or Reaper's Touch)
Your touch can heal or cause grievous wounds. You have a pool of power that replenishes when you take a long rest. With that pool, you can restore a total number of hit points equal to your level x 5.
As an action, you can touch a creature and draw power from the pool to restore or damage a number of hit points to that creature, up to the maximum amount remaining in your pool. This feature has no effect on undead and constructs.

If you have Channel Divinity or Lay on Hands from a class feature, these effects presented in this background are treated as a separate entity from those class features.

Opposing Energy Weakness
Whatever alignment you chose for the background, you suffer an additional 50% from the opposing energies that come into contact with you. This extra 50% is calculated after you either fail or succeed your saving throw.
Example 1: You are negative energy aligned. A positive healing spell from a cleric would harm you, not heal you. If it would heal you for 6 points of damage instead you take 9 points of damage.
Example 2: You are positive energy aligned. An evil cleric casts inflict wounds and rolls 3d10. They roll a total of 18. You would instead take 27 damage from the spell that opposes your alignment.