What is a word for a character who incites the conflict in a plot?

I realize you can use 'antagonist' but I'd prefer a stronger word with virtually the same meaning. I want to use it to describe Nils Krogstad of A Dollhouse.

Northrop Frye, in The Anatomy of Criticism (1957), introduced the term "blocking character" for the character who prevents the hero of a comedy from realizing his goal—typically marriage to the heroine—and associated the role with the alazon or 'braggart' of the Tractatus Coslinianus. This analysis enjoyed considerable popularity in the 60s and 70s and appears still to be widely taught.

Under the conventions within which Ibsen operated Krogstad is indeed the overt blocking character, the impediment to Nora and Torvald's marriage. But of course Ibsen is working to subvert the comic conventions of his day; and in the third act he effects an ironically inverted anagnorosis, a climactic transition from ignorance to understanding, in which Krogstad is revealed to be in fact the classical eiron, the 'self-deprecator' who contrives the stratagems which effect the 'happy' ending, and Torvald himself is revealed to be the actual blocking character and braggart.

Try instigator:

A person who brings about or initiates something:

EXAMPLE SENTENCES

he was not the instigator of the incident

any instigators of violence will be punished

The grizzled man is the instigator of all the troubles between Billy and the master-at-arms.

The author thus implies that the means to understanding the causes of the violence and the motivations of its instigators lie in the study of the previous fourteen centuries.

Can architects become instigators of greater disaster-response collaboration?

(Definition and examples from Oxforddictionaries.com)

If you need to go stronger than antagonist, you could always try

nemesis n ...
3. An opponent that cannot be beaten or overcome.
4. One that inflicts retribution or vengeance.

TFD

Catalyst.

Someone or something that encourages progress or change.

Economic development and integration are working as a catalyst for peace.

(wiktionary)

It describes an agent of change, which doesn't necessarily imply conflict. But it also describes certain chemical reactions -- that could give the overtone of strength you're looking for, depending on context.