The Mad Scientist/Alchemist Build
While Crusader Kings III doesn't have a literal "mad scientist" profession, this build emulates the pursuit of forbidden knowledge and the manipulation of the natural world through unconventional means. It focuses on the Intrigue lifestyle, particularly the "Schemer" and "Assassin" trees, combined with a relentless pursuit of unique artifacts and knowledge. Your ruler should have a high Intrigue skill, and ideally, an education trait like "Intrigue Education" or "Schemer." The "Schemer" tree offers perks like "A Job Done Right", which increases scheme power, and "Master Schemer", which reduces scheme discovery chance. The "Assassin" tree, while more direct, can be used to eliminate rivals and secure your lineage through less savory means.
The "mad scientist" aspect comes from the pursuit of unique events and artifacts. Certain events, often triggered by high Intrigue or specific character traits, can lead to the discovery of alchemical recipes, forbidden texts, or even the creation of potent elixirs. While these are often randomized, a high Intrigue character has a better chance of encountering them. Seek out courtiers with high Intrigue and consider employing them as your Spymaster. They can help uncover secrets and discover new opportunities. Don't shy away from using the "Murder" scheme, not just for political gain, but to eliminate those who might stand in the way of your research or who possess knowledge you desire.
Consider traits that boost Intrigue, such as "Cunning", "Deceitful", or the rare "Genius" trait which can be applied to any lifestyle. Traits that increase stress gain can also be beneficial, as the Intrigue lifestyle often involves morally ambiguous decisions that can lead to stress. The "Seek Counsel" decision can sometimes yield interesting results, and engaging with characters who have unusual traits or backgrounds might unlock unique interactions. While direct alchemical crafting isn't a core mechanic, the pursuit of knowledge, the manipulation of others, and the acquisition of rare items through intrigue will give you the feel of a clandestine researcher.