Form of government (-archy/-cracy) where the strongest rules

I'm doing some research for a story and I'm trying to find the correct word for a form of government where the best fighter or the strongest person leads the rest.

The usual words for forms of government are derived from Greek words suffixed with -archy (ἄρχω, árkhō, "to rule") or -cracy (κρατέω, krateo, "to have power over"), as in:
- Monarchy (rule of one)
- Oligarchy (rule of few)
- Plutocracy (rule by the wealthy)
- Theocracy (rule by God)
- Anarchy (without rule)
- etc.

The only existing word that is close to what I want is 'Stratocracy' (from στρατός, stratos, 'army'), which is rule by the military, but that seems like a different concept.

The Greek word for power is kratos, but 'Kratocracy' seems like a ludicrous word ("power power"). 'Might' or 'Strength' may be more appropriate, but that would be something like dynamis -> 'Dynamocracy', which also sounds silly.

Is there a proper word for this form of rule? If not, what would be an appropriate word to invent, in line with the current convention?

According to wikipedia, Kratocracy is, actually, the word you're looking for. It's not much of an article (as of the time of this posting), but it says the definition is:

Kratocracy, (from the Greek κρατερός krateros, meaning "strong"), is [...] government by those who are strong enough to seize power through force or cunning.

The references list the Dictionary of Philosophy as the source.

Greek for physical force (or violence, e.g., rape) is βία, but that root does not seem to serve as first element in any English derivatives, per OED. (Biarchy exists but denotes “dual sovereignty, government by two”—i.e., not bia- + -archy but just bi- + -archy.)

But in Greek political philosophy, those who argue for rule by the stronger (like Thrasymachus in Book I of Plato’s Republic, and Callicles in the same author’s Gorgias) are arguing for that polity which goes by the name of tyranny, as exemplified (in Gorgias) by the régime of Archelaos of Macedon.

Have you considered autocracy?

autocracy: Government by a single person having unlimited power; despotism; a country or state that is governed by a single person with unlimited power.