Mystery: From Celestial Venerable's Backup to Red Priest

Good news: Time travel
Bad news: I've been transported to the world of Lord of Mysteries.
Good news: I am familiar with the plot related to the mystery, although not completely.
Bad news: It seems I'm just a backup plan for the Celestial Venerable, and I have a fundamental conflict with the protagonist; it's a matter of life or death.
But none of that matters. What matters is that he was forced to drink the "Assassin" potion right after he transmigrated.
Two bottles still!!!
(This book only covers and utilizes the settings from Lord of Mysteries, and will not cover The Ring of Fate.)