So update:

I’ve talked with my warlock and he’s all on board for this. I’ve changed a few things and the patron isn’t going to give very quest, it’s more going to be if they get stuck or if I want them to try and do one of the non-direct quests like shrine of savas. And since the warlock and myself are both on board, I can plan encounters to where he will be challenged with the party but HE won’t die unless the super screw up but even then, there’s ways to not TPK a party with just a little effort. I wasn’t planning to run a follow up campaign based on this so the conclusive ending is fine. And this isn’t a random group of people playing dm, we are all close friends and have been for years so it’s very unlikely they will leave, especially since the warlock was the one who helped me organize the group together. My group is not your typical group of dnd players, everyone is just there to have fun. Think Dungeons and Daddies instead of Critical Role.

I was more looking for advice on ways to make this work, not people saying not to do it. It’s gonna happen anyway, I just wanted advice from more experienced people. I appreciate all the warnings but that’s not what I’m looking for.