What is this fairing on a Cessna 421?
That “fin” is not there on a stock Cessna 421. It's part of the RAM modification  as pointed out by Mike Sowsun. That fin corrects airflow over the wing (probably at high angle-of-attack conditions) which is perturbed by the presence of the fuselage and thereby delays the stall for that portion of the wing near the root. In essence, it "tricks" the portion of the wing near the fuselage into thinking that the fuselage isn't there. 

I invite anyone else with information on the RAM mod package to add more detail in a comment and I will edit this answer accordingly.