Creating your D&D worldbuilding can be the biggest challenge. After all, it's a role-playing game: it's supposed to give them the opportunity to do whatever they want. And maybe they will want to head away from the plot points and check out what's on the other side of the hill. 

As you hold the spluttering torch overhead, you press a cloth to your face and lean in to read the inscription on the wall - after all, you don’t want to breath the air down here freely. Welcome to the tunnels and chambers of the Undercity. 

This is the Realm of Faerie, like a dream come to life. And where there are dreams, there are nightmares. 

You are walking through a darkened hall, tall pillars rising ominously around you to a shadowy ceiling lost in darkness. A steady drip echoes through the vast, cold space, the sound sharp and lonely in the oppressive silence. With each step, the hairs on the back of your neck stand on end, and a prickle of unease tells you that unseen eyes may be watching from the shadows. Welcome to the world beneath the surface, to the depths of a Megadungeon.

When writing war for your fantasy world, it may be easy to think only of one kind of war. Of weapons and armies, marching towards one another on the field of battle. But unconventional war - war of different means and motive - can be far more involved and compelling.