"principal instigator" is correct and usable in written English.
You could use it to refer to someone who is the primary cause of something, for example, "The police believe that the principal instigator of the riots was the local gang leader."
The Jamaican assembly, dismayed, ceded its power to Governor Edward John Eyre, who declared martial law, suppressed the rioters, and hanged the principal instigator, Paul Bogle, and his alleged coconspirator, assembly member George William Gordon.
Xiao Jianhua was sentenced to death as the "principal instigator" in the factory melee that pitted a group of Han Chinese against a group of Turkic-speaking Uighurs from the Xinjiang region.
At a trial on 10 October at Shaoguan Intermediate People's Court, Xiao Jianhua was sentenced to death for being the "principal instigator" of the violence and Xu Qiqi was given a life sentence for manslaughter; three other people were sentenced to seven to eight years for assault.
It does not hesitate to refer to the incarcerated prefect as "one of the principal instigators" of the fire, without bothering with such niceties as "alleged" or "suspected".
Fine particulate matter (PM 2.5) is the principal instigators of adverse health events, yet gaps still remain in understanding the mechanism mediating its toxic response.
Laurent Gbagbo gained recognition as one of the principal instigators of the student demonstrations during the protests against Houphouët-Boigny's government on 9 February 1982, which led to the closing of the universities and other educational institutions.
They said that the encounters were organised around his busy international schedule, that he was the instigator and principal beneficiary and that the type of sexual practices involved and the lack of use of condoms suggested the women participating could only have been paid.
The principal investigator?
The cover of the next issue of Life featured not King but the instigator of the march, the labor leader A. Philip Randolph, and its principal organizer, Bayard Rustin.
At a recent panel discussion called "Archives as Instigator" at the Museum of Modern Art, other likeminded obsessives, including data artist Jer Thorp, writer Emily Spivack, code artist Mario Klingemann and MetaLab principal Yanni Loukissas, discussed the art of massive data.
The instigator was Fred Copeman.
