DM Academy

We had a campaign where something like that played out.

The warlock in that case wasn't the traitor (although he was kind of evil and ruthless), but a paladin in our group was. He built his character to spy on and eventually assassinate the warlock, which made sense in the story. The paladin coordinated this with the GM, although not with the other players. Over the early campaign the warlock saved the paladins life on multiple occasions. The paladin felt super guilty and came clean to the warlock, who asked his patron to chain the paladin to him and make him his loyal servant. Again, GM coordinated with all involved and henceforth the paladin took psychic damage when he acted against the warlock, didn't follow his express orders or strayed too far from his side. He also got a few cool abilities out of it, him being a warladin (or pallock?) now and all.

This made for a great story, but basically killed the game. It took away the agency of one player and we found no way to resolve it neatly. The warlock and warladin players made new characters and the dark duo became our BBEGs.

If you want to do this, have a clear and defined exit strategy. Make it clear to the traitor player that his character will be killed if no other solution is found during the campaign. In this scenario the experience of the rest of the group is more important because they did not consent to this.

If you want to do a cool double twist, you could do the following. If he reveals himself during the final showdown to be a traitor, have the BBEGs sidekick also be a traitor. The two basically switch sides, the player switches characters and continues to play with the former BBEGs sidekick.