Vile damage came from an earlier edition of the rules, and was damage so unspeakably evil that the only way to heal it with magic was to do so while in the area of effect of a consecrate, hallow, or similar spell effect.

Now, adding a completely new damage source isn't really something 5th edition needs right now, but the theme is certainly appropriate. So, this update:

Vile damage: This necrotic damage is so unspeakably evil that it can only be healed while in the area of effect of a consecrate, hallow, or similar spell effect.  

Vile spell (Metamagic): Whenever you cast a spell, you can spend a number of sorcery points equal to the spell's level to change the spell's damage from the normal damage type to half-normal, half-vile. Thus, a fireball doing 27 points of fire damage would cost three sorcery points to turn into a vile fireball,  doing 14 points of fire damage and 13 points of vile damage.

Vile enhancement (Feat): When you gain this feat, choose one of the following damage types: acid, cold, fire, lightning, or thunder. Spells you cast with the chosen damage types can instead do half the chosen damage type, and half vile damage.  You can select this feat multi pIe times. Each time you do so, you must choose a different damage type.

Lifequencher (Invocation) [Prerequisite: 15th level, Lifedrinker invocation]: When you hit a creature with your pact weapon, the creature takes extra vile damage equal to your Charisma modifier +1 (minimum 1).

This makes resistance / immunity to necrotic damage a more valuable tool in the game, since anything that works on / against necrotic damage also does so for vile damage.

It also enhances the value of having a PC who can cast consecrate, hallow, or similar spell effect in the party.

Additionally, it gives a DM a chance to make all kinds of custom spells. Just increase the spell slot necessary to cast it by 1, rename it, and treat it as a vile spell. So Halaku's Hateful Havoc would be a vile fireball that takes a 4th level spot to inscribe in a spellbook and to cast, and is otherwise treated as a 3rd level spell.

It can also cause significant moral dilemmas for player characters. How does their chosen religion feel about vile damage? What about other religions? How do they feel about fellow party members causing vile damage? If they recovered a spell scroll of Halaku's Hateful Havoc, would they try and sell it at a premium to an unscrupulous wizard, or destroy it? Would they be prepared to fight off anyone who wants to take it off their steaming corpse? 

Thoughts?

So necrotic = vile?...I don't understand I feel if you want this effect just change the name of necrotic to vile in your players core rule book.

Quote from BeerTitan >>

So necrotic = vile?...I don't understand I feel if you want this effect just change the name of necrotic to vile in your players core rule book.

But why instead of adding another mechanic called vile don't you just supplement necrotic damage. It would be a lot easier just say necrotic damage does this effect as well as this.

Quote from BeerTitan >>

But why instead of adding another mechanic called vile don't you just supplement necrotic damage. It would be a lot easier just say necrotic damage does this effect as well as this.

Personally, I like what Halaku has done with the concept. My only complaint is, looking back at the Book of Vile Darkness, very little vile damage is dealt by any attack. The old Vile feats only allowed one point of vile damage to be dealt with an attack, though during the course of a combat, that would stack up. In a game where massive amounts of Vile damage are dealt at a time, it becomes harder to survive, especially if fighting a powerful monster who can deal vile damage. Maybe changing the options to deal 1d4 extra vile damage or something might be more balanced.

EDIT: Just took a second look at the Book of Vile Darkness. Apparently, there are some feats that changes half the damage to vile. But still, I would be careful with how much vile damage can be dealt in a single attack.

Vile spell (Metamagic): Whenever you cast a spell, you can spend a number of sorcery points equal to the spell's level to change the spell's damage from the normal damage type to half-normal, half-vile. Thus, a fireball doing 27 points of fire damage would cost three sorcery points to turn into a vile fireball,  doing 14 points of fire damage and 13 points of vile damage.

Vile enhancement (Feat): When you gain this feat, choose one of the following damage types: acid, cold, fire, lightning, or thunder. Spells you cast with the chosen damage types can instead do half the chosen damage type, and half vile damage.  You can select this feat multipIe times. Each time you do so, you must choose a different damage type.

The feat seems much better than the spell. The feat doesn't require use of sorcery points, though it is limited to one damage type. The metamagic offers versatility in damage types, but you'll burn through sorcery points very quickly. The feat would save you both a metamagic slot and sorcery points.

I can't offer any advice on how to make them more equal, though. Perhaps do half spell level rounded down/up?

Quote from BeerTitan >>

But why instead of adding another mechanic called vile don't you just supplement necrotic damage. It would be a lot easier just say necrotic damage does this effect as well as this.

Quote from Imoxator >>

Vile spell (Metamagic): Whenever you cast a spell, you can spend a number of sorcery points equal to the spell's level to change the spell's damage from the normal damage type to half-normal, half-vile. Thus, a fireball doing 27 points of fire damage would cost three sorcery points to turn into a vile fireball,  doing 14 points of fire damage and 13 points of vile damage.

Vile enhancement (Feat): When you gain this feat, choose one of the following damage types: acid, cold, fire, lightning, or thunder. Spells you cast with the chosen damage types can instead do half the chosen damage type, and half vile damage.  You can select this feat multipIe times. Each time you do so, you must choose a different damage type.

The feat seems much better than the spell. The feat doesn't require use of sorcery points, though it is limited to one damage type. The metamagic offers versatility in damage types, but you'll burn through sorcery points very quickly. The feat would save you both a metamagic slot and sorcery points.

I can't offer any advice on how to make them more equal, though. Perhaps do half spell level rounded down/up?