So I’m world building and one of the region BBEGs I wanted to make a pure barbarian but have access to some spells. I know as the GM I can do what I want but I need to make sense personally. 

one idea was he was a secret acolyte of Vecna with Vecna gifted him with the Book of Vile Darkness. The lore section of the book says it has access to all evil spells/rituals. Would it be plausible for said barbarian to cast spells from the book even though no spell slots (they would all be story line spells not direct combat related) 

if not how would you as a GM do this?

I don't know the level, etc. of your game but introducing the Book of Vile Darkness is a BIG thing.

Surely it would be easier just to have something in his background that gave him this ability - much like players can get Fey or Shadow Touched?  Or  a patron allowed him these abilities (like a Warlock)?  Obviously, make it appropriate to your Barbarian (Auril if in the North, for example).

Not sure if that helps, but that's how I would 'justify' it.

Quote from mcdaygo13 >>

So I’m world building and one of the region BBEGs I wanted to make a pure barbarian but have access to some spells. I know as the GM I can do what I want but I need to make sense personally. 

one idea was he was a secret acolyte of Vecna with Vecna gifted him with the Book of Vile Darkness. The lore section of the book says it has access to all evil spells/rituals. Would it be plausible for said barbarian to cast spells from the book even though no spell slots (they would all be story line spells not direct combat related) 

if not how would you as a GM do this?

Casting a spell from an item (including the Book of Vile Darkness) doesn't use a spell slot, so I don't think this is a problem.

Quote from Holton_Coalfield >>

I don't know the level, etc. of your game but introducing the Book of Vile Darkness is a BIG thing.

Surely it would be easier just to have something in his background that gave him this ability - much like players can get Fey or Shadow Touched?  Or  a patron allowed him these abilities (like a Warlock)?  Obviously, make it appropriate to your Barbarian (Auril if in the North, for example).

Not sure if that helps, but that's how I would 'justify' it.

 I agree it is huge but he is meant to be a CR 20 encounter. Game will run 1-20

Creatures do not need to follow the same rules as a player - reskin a an ancient dragon or something as your boss add what you want and be done with it - its a dragon/giant/demon/celestial/ whatever  with a quirk for fighting in his barbarian aspect. 

Also, keep in mind that items like Enspelled Weapon and Helm of Teleportation exist, so you could always give your NPC some magical goodies.

You could always just re-flavor the spells as an expression of his connection with nature spirits and have it spend a rage usage to power it. Like how they can use rage to boost skill checks now. 

Id also offer the standard advice of not using a PC to make an NPC, as they’ll have lots of extra stuff you don’t need, and are typically not going to be very strong against other PC. Monsters have high hit point numbers for a reason.

And if you’re in the world building phase, I wouldn’t be starting out individual NPCs yet. The players may not even meet the guy. Have an idea, but keep things vague for now. 

Quote from James_Bard >>

Creatures do not need to follow the same rules as a player

This is the correct answer. If you want your "barbarian" BBEG to cast some spells, then they do, and you don't need a reason for it beyond Evil. You can always hand-wave it away with some clue before they start slinging spells if you want -- maybe the BBEG has tattoos that an Arcana check will recognize as magic, that sort of thing -- but you really don't need to introduce some deep lore to explain it, or shoehorn powerful relics into your campaign just to cover up a plot hole that isn't even there

If you, personally, want the villain's backstory to explain how they can cast spells despite outwardly appearing to be a "barbarian", then do that because it will probably make for a better villain, but you don't need a mechanical justification for it. It only needs to fit with the story you're telling

I mean, is Vecna already a big deal in your campaign? If not, then why introduce him here? That's generally not a name you drop casually into the plot without your party thinking he's going to become THE PLOT down the road

He inherits the Power of his Giant Ancestors. Hand wave the rest...