I'd go with a bio-weapon.
A decomposing fungus was engineered to produce high-persistence non-selective herbicides. You spread it to an area, it kills the plants using the already dead plants for energy. Then your enemy has no agriculture and either starves or buys food from you. Simple and brilliant.
Or as it turns out, the enemy may simply collapse creating a huge influx of refugees into your area. Refugees that spread with them your bio-weapon wherever they go. Bio-weapon now doubly brilliant as the enemies efforts to kill it had provided it with resistance to all fungicides known to science, including the one you had specifically engineered it to be vulnerable to as a safe guard.
You fungus becomes a global phenomenon killing plants world wide almost faster than your agents can cleanse evidence of you ever having had anything to do with "the plague". Fortunately people are too preoccupied fighting over the remaining food stocks and fishing fleets to investigate. Dying from starvation is pretty good distraction too.
You start thinking about the ways you can turn this to your advantage. With so many people dead and so much agriculture lost, the world will be redrawn. This is a huge opportunity to anyone with some initiative and enough common sense not to waste resources trying to save people who cannot be saved. You start to think this was a blessing. Maybe your subconscious was able to see farther ahead? Maybe ruling the world is not your ambition but your destiny?
Then things start to go wrong. With plants dying all over erosion accelerates. Huge amounts of top soil is carried to the oceans by wind and water. At which point we find out that the herbicides produced by the fungi can kill most forms of algae if there is enough of them. And there is.
Not enough to seriously threaten the algae themselves, mind you. But combined with combined with starving people fishing all they can, the sudden collapse of the algae pretty much wipes out the fish from the ocean as a viable food source.
And things keep getting worse. Turns out that if you kill enough plants they turn less carbon dioxide to sugars and oxygen. And that poisoned plants and starved animals release carbon dioxide to atmosphere as they decay. The greenhouse effect gets a huge and rapid boost.
The surviving people mostly just shrug their shoulders in response to this. Does it really matter any more? Turns out it does. The increased temperature creates torrential rains and huge arid areas. With most terrestrial plants dead, the erosion accelerates to speeds never seen before.
Thanks to high persistence herbicides in the soil and high levels of carbon dioxide persisting this goes on for centuries. Apart from few isolated pockets the top soil is lost and the landscapes become barren and arid.
But there is good news, those herbicides took lots of energy to produce so the fungi evolved not to bother after few decades. And most spores of the old strain died after few centuries, as well. Problem solved.
Marine life rebound after a century as well. Even the stablest poisons became safely embedded in the sediments and algae recovered completely. The fish more than recovered. With the most people dead and even the survivors unable to really build ships, humans no longer are a threat to fish flourishing.