The PCs traveling along the road/path/trail begin to hear a low droning sound. As they approach they find their path obstructed by hundred of thousands of droning insects. Locusts swarm though the air, feasting on the grass and plants. Passage will be slowed, but the insects pose no threat to the party, aside from being crunchy and disgusting underfoot, climbing into pouches and eating exposed food goods.
Appearing a small ballista bolt, this ‘bolt’ is actually an arrow used by a god from a long-dead pantheon. Some sages theorize it was a weapon crafted by Loki to slay the world tree. Why it was never used escapes them however.
The spear has a divine-level anti-plant effect. When driven into the soil point first, a ever widening circle of destruction radiates out from the point, killing any plant material. Perfect for holding a kingdom’s food supply for ransom.
How far it extends depends of course on the GM, of course.