At first glance, the plot would seem to be:

When a brutal warlord kills her father, a sheltered princess must take over as queen and defend her kingdom against a barbarian invasion.
(23 words)

As if the death of her father and the invasion are 2 separate crises by 2 separate parties.

But if the same warlord is also the invader — his goal is to claim country and throne for himself — then perhaps:

When a brutal warlord kills her father and invades the kingdom, a sheltered princess must fight for her throne and the freedom of her people.
(25 words)

A virtue of the 2nd version is that she is fighting for something greater than just herself; she’s fighting for her people, for freedom– more important stakeholders and stakes than her claim to the throne.? [That’s not an insignificant nuance in the way historical (and fantasy) stories are framed these days, btw.]

Whatever, the ambiguity as to the role of warlord in the entirety of the plot needs to be clarified.

Also, I believe it always more effective to cast a character’s action — even the antagonist — in active, transitive words.? So “a brutal warlord kills the father” rather than “the father is killed by a brutal warlord”.? ?It also saves 2 words. achieving? economy as well as effectiveness.

fwiw