When you enter a monster’s lair, you’re in its home. It should have home-field advantage.

1. Dark Corners

The creature is comfortable in the dark. Even if the heroes have torches, they don’t reach into the corners. In a large cavern, they’re effectively in a pool of light; the creature can move from shadow to shadow & spring from seemingly anywhere.

2. Difficult Terrain

Whether it’s webs, slippery stones, hanging vines, loose gravel, plant roots, or some other tricky footing, the monster may be comfortable with terrain that is difficult for the heroes, slowing their movement or risking a fall.

3. Pool of Water

The creature can dive into the pool without fear, because it knows the pool isn’t filled with acid or something. It can swim and knows how deep the water is and if there’s something dangerous at the bottom.

4. Flooded Chamber

The whole chamber is flooded to a depth of a few feet, but the creature is big enough to wade thru it easily, maybe even lie down with its head still above water, especially if there’s a little underwater hill that creates a shallow spot. If you want to attack it, you’ll be wading waist-deep. And maybe there’s an extra deep spot you can’t see….

5. Hillock

There’s a little hill in the cavern that the creature can climb onto, which gives it advantage over any heroes trying to approach.

6. Ledge

There’s a ledge out of reach of the heroes that the creature can leap up to or pounce from. There may be no easy way to get up to it, if the heroes can’t fly. A lower ledge would act like a hillock, giving the creature advantage.

7. Mist

A waterfall in or just outside a cavern can create a permanent mist that obscures vision. Smoke or steam in a hot environment can produce the same effect.

8. Fire

If a creature has fire resistance or immunity, there’s a good chance it likes to live near fire. Maybe there is volcanic activity in its lair or flammable materials that could create an inferno if someone with a torch isn’t careful.

9. Trap

The creature knows where the trap is. It may even have set it. Maybe it’s a weak spot in the cavern floor that could collapse. Maybe it’s a patch of deadly yellow mold the creature can avoid or is immune to. Maybe it’s a pit covered with sticks and disguised with dirt and debris.

10. Deep Freeze

The creature isn’t bothered by cold & so lives high in the mountains in an icy cave. It’s not only cold, it’s often slippery.