Someone who deliberately foments trouble:
— She was the instigator of their quarrel.

synonyms: firebrand, inciter, instigant, provoker.

Roget 615: motive, springs of action, wellsprings of action.    reason, ground, call, principle; by end, by purpose; mainspring, primum mobile [Lat.], keystone; the why and the wherefore; pro and con, reason why; secret motive, arriere pensee [Fr.]; intention etc. 620.    inducement, consideration; attraction; loadstone; magnet, magnetism, magnetic force; allectation^†, allective^†; temptation, enticement, agacerie^†, allurement, witchery; bewitchment, bewitchery; charm; spell etc. 993; fascination, blandishment, cajolery; seduction, seducement; honeyed words, voice of the tempter, song of the Sirens forbidden fruit, golden apple.    persuasibility^†, persuasibleness^†; attractability^†; impressibility, susceptibility; softness; persuasiveness, attractiveness; tantalization^†.    influence, prompting, dictate, instance; impulse, impulsion; incitement, incitation; press, instigation; provocation etc. (excitation of feeling) 824; inspiration; persuasion, suasion; encouragement, advocacy; exhortation; advice etc. 695; solicitation etc. (request) 765; lobbyism; pull [Slang].    incentive, stimulus, spur, fillip, whip, goad, ankus^†, rowel, provocative, whet, dram.    bribe, lure; decoy, decoy duck; bait, trail of a red herring; bribery and corruption; sop, sop for Cerberus.    prompter, tempter; seducer, seductor^†; instigator, firebrand, incendiary; Siren, Circe; agent provocateur; lobbyist.    ... show more

Dutch: uitlokker
Polish: judziciel, podjudzacz, podżegacz

A person who initiates a course of action.

synonym: initiator.

Dutch: initiatiefnemer, initiator
Polish: duchowy ojciec, inicjator