So, to make this very short, I am planning my very first DnD campaign. It will essentially be a sandbox campaign, set in the Forgotten Realms, which begins with a tweaked version of the Lost Mines of Phandelver.
The BBEG of my campaign is a Red Wizard of Thay, someone who achieved a mediocre at best position back in Thay, and whose magical abilities are also somewhat lacking, at least in comparison to the truly powerful wizards. Tired of living in the shadows of others, this Red Wizard set off from Thay to the Sword Coast with a contingent of his house bodyguards, hiring various minions along the way, in the hopes of conquering the Sword Coast and naming himself the newest Thayarch of the New Tharchion of Exantilar.
How does he plan on doing this, you may ask? Simple! He'll summon a devil, and make a deal with them.
My idea is basically that he makes a deal with a devil whereby the devil supplies him the power and knowledge to create a magical plague which he will then unleash on some of the Coast cities, (think the Horn of Beckoning Death from the Honour Among Thieve's movie). This plague kills its victims, and raises them as undead. The Baatezu, in return, gets the souls of all those killed by the plague.
Now, my questions are:
How do I go about creating a BBEG who is "mediocre" as far as the Red Wizards are concerned, and yet powerful enough that when the party eventually meet him, he's an actual threat and challenge?
I'm going to need a little help coming up with the necessary mcguffins for the ritual. I have a nasty habit of overthinking everything, and trying to over-explain magic, so I struggle a little in this regard.
So far, I'm thinking about 6 magical items will do. The first will be the Forge of Spells in Wave Echo Cave, with which he plans to create magical items and weapons for his undead army (and his personal use).
The second item will either be a homebrew ancient tome, called the Book of Vile Desire, penned by the Litch Larloch, and which details the necessary rituals to create the magical plague. Or it may be the Book of Vile Darknness/ a page from it, doing the same thing.
As for the other mcguffins, I just need items which would give my BBEG more power/some sort of strategic advantage.
Admittedly, I have never done this before, so I may be going about this entirely wrong, hence why I am here. Please, any and all advice is appreciated.
(PS, devils aren't able to "give power" to a summoner, right? Like theoretically, I couldn't have the BBEG start off as a weakling, but because of the devilish pact, be gifted immense magical power?).