I realize they is touching on Warlock territory but hear me out. A character who takes a Dark Bargain and ends up with a symbiotic being that happens to be a fallen angel/devil. This being is essentially the character magical tutor/teacher. The symbiotic being whispers in his ear, instructs him how to cast spells and provides instruction on magical matters via a mindscape similar to the dreams and thing in the heads of the characters in Baldur’s Gate 3 or Lasciel’s shadow in Harry Dresdens mind in the Dresden Files. What do you guys think? Is this a good idea or is it too far in the warlock realm?

I’ve always believed that Wizards are perfectly capable of pulling off the “darkness” or “eldritch” feeling.

Truth be told, I’ve been patiently waiting for a Wizard subclass that can adequately encompass “dark” magic, like curses or the occult.

Anyway, for your concept, I would probably recommend either the Enchantment or Illusion options for the Arcane Tradition.

Enchantment offers some cerebral or psychic options which fit the mind-bending of such a character.

However, Illusion spells also allow you create the “dreamscapes” you might desire…specifically, the “Dream” spell achieves the effects I think you’re after.

A Dark Bargain with a symbiotic being is specifically allowed for any character class via Van Richten's Guide; so it'll definitely work! I love me a Wizard who gets taught dark/occult magic, especially if they themselves straddle that gray area between good and bad - there's a lot of fun examples of similar things in media. I say go for it! You already have a great backstory hook for the DM right there.

I'd go Order of Scribes and have a Necronomican spell book and then you can have some conversations with your "patron". I;d also mention a wizard could technically make a spell so ask your DM if you can research your ow spells or copy those from other spells lists.

There are some really great ideas here. Each reply adds some great flavor to the idea. 

So for me when Jessebruffett mentioned the wizard with a terrible host, it immediately made me think of Percy de Rolo of the Vox Machina series. Demons are notorious creatures of meddling with mortals. They will provide knowledge and power for a very hefty cost. While some of your more powerful demons can offer the "Warlock" quick path to power. Your lesser demons can tempt mortals with power and knowledge. This could even be a gift to your DM to add more flavor or even a complete side quest or story. If I were DMing this story, I would allow any casting class to take the Pact of the Tome to provide a bit of flavor, but it would cost you a feat. Also I would expect at times for your character to lose control at major plot points that might bring out your demonic guest depending on how the DM makes this work. 

But I think your idea is brilliant and will really test your ability to play moral conflicts in a good story. And your contact between the entity and your character will something between you and your DM to setup. There really is no wrong way to do that. 

I appreciate the info! I think im gonna steer clear of Order of the Scribes though, I loathe that subclass. I thought about multi classing wizard and warlock but that always seems like a rough build and I don't like straight warlocks, they always seem so limited in spell casting because it always seems like short rests just rarely come up in games, especially with the dm whose leading the game we are starting. I just wasn't sure overall if the whole devil/demon sharing a mindspace and body and being an arcane teacher would be too much encroachment on the warlock or not. The game is gonna start at level 3, Im, going variant human with eldritch initiate as the free feat kinda as the way he started down the path to being possessed-ish by the devil and devils sight is a great invocation if you don't have darkvision. Im flavoring it as my characters eyes will either have black sclera like tieflings or he will have cat eyes. We aren't having the session 0 until next week so if anyone else has any ideas or advice please don't hesitate to comment.

Fair comment on Order of Scribes it seems to be a love it or loathe it subclass.

Have you considered going for a mashup? See if your DM will allow you to play a wizard but port over the Fiend Warlock powers? That would mean you get Dark One's Blessing at level 2, Dark One's Own Luck at level 6, Fiendish Resliience at level 10 and Hurl Through Hell at level 14, that wouldf be thematically explained as a alternative pact and you have the trade off of not being able to scribe spells on the cheap and also plays into a more Demonologist/Diabolist/possessed wizard type.

I talked with my DM earlier and we went over this concept and build idea from a couple different angles. He said that he would HB warlock as an INT caster and if I wanted, and he is encouraging but leaving it up to me, that I can take 3 levels of warlock and pact of the tome and flavor the book of shadows as my wizard spellbook for wizard levels after that.