Derren said:
Silent Image placed over a very deep pit for example.
Of course the dragon must know that the adventurers are coming to do this. Thats what the alarm spell is for which in itself is already a trap.
Most traps I create for monsters don’t perfectly mimic spells. I don’t need to explain to my players how the blue dragon managed to set up a permanent rain storm in the cavern passage. When they entered storm, they found it zapped one of them randomly with lightning and that thunder rumbled throughout the cavern, alerting everything within that someone was passing through the storm. They marveled at it, attempted a dispel magic (which failed miserably), and then warded themselves against the electricity damage and passed through. There are no spells to create this kind of effect in a cave, but there it was.
Making something unique gives this dragon’s lair flavor and shows the innate power of the dragon. My players don’t scoff at it and say it’s against the rules, so I can’t do it. They don’t say that blue dragons don’t command such power. They appreciate the time and effort I take to craft an adventure and the challenges it presents and accept the fact that there are things beyond their scope. They may be powerful, but they are not all-powerful.
I want the monsters to be well statted for combat. I need to know how to balance a fight for my players to be challenged. I don’t need WotC to tell me what a creature like a dragon is doing in its spare time. If I am placing a dragon in a campaign, I am doing so with a purpose and I know what it is doing in its spare time. Out of combat spells and abilities (such as alarms, traps, and lair crafting) I can, have, and always will fluff up on my own as I see fit.
My players would never say “Dragons don’t cast spells anymore so how does this one have magical traps?” and even if they did ask it, I would tell them that their character’s arcane knowledge is good enough to know that, while dragons are not sorcerers, they are still highly magical creatures that can bend magical powers to their will, given enough time and concentration, something they cannot muster in the middle of combat. Just because WotC didn’t write that in a MM doesn’t mean it isn’t true.
I’d rather have complete and balanced combat stats and make up my own non-combat stuff than have twice as much in a stats block, a full list of everything a creature can do, and ignore it because the out-of-combat fluff didn’t fit my needs anyway.