Keen Smell and Illusions: the Forgotten Sense

[RickAllison]

So here is an interesting question. Can a creature with Keen Smell (such as a werebear) really be effected by most illusions? Most of them rely on sight and/or sound, but wouldn't any visual illusion be disregarded when it doesn't smell any different? This thought randomly came to me and I would like some input from the community.

[Felvion]

I think you have a point, i'd go with circumstancial advantage. One could give advantage on the saving throw to a creature with keen smell as long as the illusion is supposed to produce (or reduce) a strong odor.
For example, let's say a wererat is chasing a halfling illusionist in a labyrinth with no particular smelly distractions. The wererat takes a turn and suddenly sees a horse (actually just the image of an illusory one) in front of him. It's safe to say he has advantage to disbelieve the illusion cause he's a sentient being that knows horses smell a lot. After a while he reaches a dead end. The corridor is not really over but the halfling casted an illusion to "create" a wall. Keen smell doesn't help against a wall, walls don't smell so no particular advantage. On the other hand, if the halfling is standing right behind this "wall" all sweaty and exhausted he probably smells enough to give the wererat a hint there's something wrong in this corridor...

[Madbox]

I agree with Felvion here. Don't forget about other senses for illusions as well. For example, a creature with tremorsense or blindsight might not fall for a visual illusion, as it doesn't make vibrations in the earth or respond to echolocation or something.

[Regitnui]

The converse of this is that illusions, especially major images, can be altered to produce senses other than sight. Of course, it'd be difficult for a humanoid to make an illusion of a smell 'correct' to fool a dog, wolf or Keen Smell user. It'd also be almost impossible to fool a tremorsensing or blindsighted creature unless the crafter of the illusion had sufficient experience with the sense they're trying to fool.

[Kurt Kurageous]

I think you have a point, i'd go with circumstancial advantage. One could give advantage on the saving throw to a creature with keen smell as long as the illusion is supposed to produce (or reduce) a strong odor.

Keen smell doesn't help against a wall, walls don't smell so no particular advantage.

Respectfully, I would rule the wererat advantaged if in rat or hybrid form if it got within 30' of the wall.

Walls (like everything) do smell different than empty air. It doesn't have to a strong odor to a human nose. What is faint to a human nose is overwhelming to a canine or other keen smelling creatures. It is difficult to describe how incredibly inferior our human olfactory senses are compared to a dog, but a quick google will give you an idea.

A second roll of a d20 is nowhere near a fair ruling. Closer to fair would be a hundred rolls, IMHO, but that's not an option.

I try very hard to make illusions worth casting in my game. But I would rule this illusion in this circumstance consumes one reaction/bonus action of the 'rats turn, but deny it dash as its action that turn.

Perhaps something out of place would be better. How about a figurine in the form of a rat goddess/god, or a very large mousetrap? It would arouse the curiosity and cause the 'rat to slow/stop to examine it, then be confused by the non-smelly nature, then disbelieve it. I'd give you a whole turn of delay for that.

How much more do you really expect from a cantrip?

[Captbrannigan]

How much more do you really expect from a cantrip?
Nitpick: No one said Minor Illusion.

I would simply say that Keen Senses make them more likely to spend a round investigating, just like the image of a flame with no heat radiating is not an auto-success but isn't likely to fool anyone for long.

My experience is that illusions are generally too weak or too strong, depending on how your DM rules. They either mostly work for a single turn or enemies ignore them, or they cripple whole encounters and are the best CC in the game.