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I have my PCs entering a mansion owned my an illusionist-rogue type of BBEG. He's going to be casting Minor Illusions, as well as Hallucinatory Terrain throughout the mansion to mislead players into falling for traps, walking into unfavorable scenarios, as well as getting the jump on them.

How do I implement this as the DM, without making it entirely unfair to the players? The spell Hallucinatory Terrain reads as such:

If the difference isn't obvious by touch, a creature can attempt to make an Intelligence (Investigation) check against your spell save DC to disbelieve it.

(PHB. 249)

With that said, if the players walk into a room, and see a finely placed dinner table, chairs, and smell and see the fresh meal placed before them (as an illusion), at what point do they realize it is all fake, and that they're truly standing on a giant trap door?

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The Angry GM has a great article on just this topic. I recommend reading it in its entirety, but here are the most relevant passages:

First of all, you have to get into the habit of lying to players. Remember, your job is to describe what the characters see, here, and perceive; NOT what is actually real. There ARE situations in which a GM is allowed to lie to the players. For example, when portraying a dishonest NPC, the GM lies because the NPC lies. And when portraying a trap, the GM lies about what’s in the room if the trap isn’t spotted. And when dealing with illusions, the GM has to lie to maintain the illusions. And that is perfectly fine. ... That also means you have to get into the habit of rolling saving throws in secret and also in knowing WHEN to roll saving throws.

...

You CAN’T run an encounter with an illusionist. An illusionist has to be the end of an adventure. Or at least, the illusionist needs a warmup act. An illusionist needs a lair. And that lair needs to teach the players that they can’t tell the difference between what’s real and what’s not. ... You can’t tell what’s real or what’s not. Neither the players nor the characters can. So, then, when they end up fighting the illusionist whose room has several pits in it, they can’t tell which pits are pits and which ones aren’t without experimentation they don’t have time for. And if the illusionist can levitate and stand in a pit OR sometimes just stands on pits that aren’t real, the PCs can’t even tell by his movements which pits are pits.

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  • \$\begingroup\$ This helps immensely, thank you for providing this. \$\endgroup\$ – DM Patman Feb 8 '17 at 21:05
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Do not forget that Hallucinatory Terrain states that:

Manufactured structures, equipment, and creatures within the area aren't changed in appearance. (PHB 249)

So sadly you cannot use that to disguise a mansion. Even if you say as a GM that your NPC has a spell capable of that, the PCs would only need one successful check to see through it in the entire building. Multiple small illusions cast using surveillance/sneaking or servants/magic items (or anything you come up with to justify it) would be better.

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