The Illusionists Guide to Reality

[Ovinomancer]

Thanks for the question.

It doesn't take a darkness spell to shut down the only light in a room. Minor Illusion could put a box around it until it was proven to be an illusion, but other cantrips can do it more effectively. Prestidigitation will snuff out a flame, as will Control Flames. Anyone can "extinguish a small flame" as part of interacting with objects (PHB p190) too without the use of magic.

If the question involved an object that had the light cantrip cast on it, the light cantrip says "Completely covering the object with something opaque blocks the light." Illusions are opaque until discovered.

Strong disagree.

Illusions cannot block anything (unless specifically noted). They have no physical reality. The notion that an illusion is enough present to block light but nothing else is special pleading.

Putting an illusionary box over a light cantrip my disguise easy identification of the source, but won't stop the light from illuminating 40' of the area.

My general rule of thumb us that an illusion can add to a scene, but cannot subtract ir otherwise alter it. This prevents oddities like illusions throwing a room into darkness because the light sorce has an image of a box around it. There's already a spell for that.

As a house rule, I've also tossed the whole "if you save you see through it as if it's not there" crud to avoid cheap illusionary one- way mirror tricks. You can recognize the illusion, but you still see it. Still makes illusion excellently useful and flavorful but cuts down on cheese uses that aren't that fun.

 

[Gadget]

Some very good points; I especially agree with the advice on getting to what the player is trying to accomplish to overcome communication issues, that is key. However, I would be careful to read the limitations of the spells in question; I've too often seen people try to get more out of a spell than it offers: like trying to create a swarm of bees or such with a minor illusion spell. I also strongly agree with [MENTION=16814]Ovinomancer[/MENTION] above that, for the most part (like the lower level spells), illusions do not change the lighting conditions, becoming a poor man's light/darkness spells. Nor are they a stationary, in place Improved Invisibility. You have other spells for that.

And while I agree that in the 'heat of battle' close scrutiny should not be enforced, things like a pool of acid could be hard to determine when it does not bubble, smoke, or eat away at the surrounding area (Minor Illusion). Without these visual and auditory clues, it is just a pool of unidentified liquid that does not move or flow like liquid usually does. Furthermore, many animals and beasts rely on senses other than sight, so things like meat to bait a trap would be a hard sell without the scent to go along with it, though I would still allow for 'heat of battle' type uses as a distraction and such.

One thing that does come up with illusions now and again is the whole 'stationary' requirement. Does that mean I can't put the image of the Baron's house on my cloak, due to the fact that the image would remain in place when I or my cloak moved? Does this prevent me from having an illusory ring on my (or someone else's) finger unless the finger remains stationary? This becomes more intriguing with a permanent Major Image and the Malleable Illusions class feature. I could have half a dozen illusory rings on my fingers that I can change to whatever illusion I desire (within the spell's parameters) when needed.

 

[Otterscrubber]

Great suggestions here, Illusions are amazing if used properly and this here was some proper advice on the topic!

 

[Lanefan]

Strong disagree.

Illusions cannot block anything (unless specifically noted). They have no physical reality. The notion that an illusion is enough present to block light but nothing else is special pleading.

Putting an illusionary box over a light cantrip my disguise easy identification of the source, but won't stop the light from illuminating 40' of the area.

Can't a part of the illusion be a lack of light elsewhere in the room?

If an illusion can generate light in the view of those who believe it (e.g. someone casting an illusion replicating said 'light' cantrip) then by the same logic it can "block" light simply by having part of the illusion's effect make it appear that there is less light. That's awkwardly put, so let me try again: if an illusion of a person, say, can include whatever shadow that person would normally cast (and a wise illusionist would include this to help with believability) then that opens the door for an illusion appearing to block light. Expand that such that a small part of an illusion is a box around a light and most of the illusion is the "shadow" that box casts and voila: the light goes out.

My general rule of thumb us that an illusion can add to a scene, but cannot subtract ir otherwise alter it.

So an illusionist in your game could not, on hearing intruders approach, cast an illusion to make her laboratory look like an empty room?

As a house rule, I've also tossed the whole "if you save you see through it as if it's not there" crud to avoid cheap illusionary one- way mirror tricks. You can recognize the illusion, but you still see it. Still makes illusion excellently useful and flavorful but cuts down on cheese uses that aren't that fun.

Why is that not fun? It's one of the best uses for illusions! Never mind the caster always has to be able to see through her own illusion if only so she can have it react properly to things behind it...

Lanefan

 

[Ovinomancer]

Can't a part of the illusion be a lack of light elsewhere in the room?

If an illusion can generate light in the view of those who believe it (e.g. someone casting an illusion replicating said 'light' cantrip) then by the same logic it can "block" light simply by having part of the illusion's effect make it appear that there is less light. That's awkwardly put, so let me try again: if an illusion of a person, say, can include whatever shadow that person would normally cast (and a wise illusionist would include this to help with believability) then that opens the door for an illusion appearing to block light. Expand that such that a small part of an illusion is a box around a light and most of the illusion is the "shadow" that box casts and voila: the light goes out.

Illusions cannot illuminate. An image of a shadow is not a real shadow.

So an illusionist in your game could not, on hearing intruders approach, cast an illusion to make her laboratory look like an empty room?

Well, area of effect solves that for all but a small room. You could certainly put an image of an empty room in the doorway, but that will fail if anyone enters. But, no, you could not effectively make everything in the room invisible. Add, as in the image in the doorway, not subtract.

Why is that not fun? It's one of the best uses for illusions! Never mind the caster always has to be able to see through her own illusion if only so she can have it react properly to things behind it...

Lanefan

Nah, it's cheese. Either an illusion is an image or it isn't. Not being able to cheese the two-way wall doesn't reduce the creativity and usefulness of illusions. I love illusions. Not a fan of cheese.

 

[Gadget]

So an illusionist in your game could not, on hearing intruders approach, cast an illusion to make her laboratory look like an empty room?
Lanefan

I'm not Ovinomancer, but I think it would depend on: what spell she was using and what is meant by 'empty room.' If she just wants to avoid discovery by the intruders she could make a cabinet or wardrobe around herself standing or sitting in a corner, have an illusory wall angle just so around her, etc. This could be conceivably covered by any of the minor, silent or major images (though she would have to take the duration into account). If she wanted to disguise that there was anything in the room at all (i.e. all the books, tables, cabinets, beakers, cauldron, shelves, etc.), that sounds more like Hallucinatory Terrain than the image spells.

 

[Swarmkeeper]

Can't a part of the illusion be a lack of light elsewhere in the room?

If an illusion can generate light in the view of those who believe it (e.g. someone casting an illusion replicating said 'light' cantrip) then by the same logic it can "block" light simply by having part of the illusion's effect make it appear that there is less light. That's awkwardly put, so let me try again: if an illusion of a person, say, can include whatever shadow that person would normally cast (and a wise illusionist would include this to help with believability) then that opens the door for an illusion appearing to block light. Expand that such that a small part of an illusion is a box around a light and most of the illusion is the "shadow" that box casts and voila: the light goes out.

Minor Illusion being a 5’ cube limits the ability of it to darken an entire room. It would just awkwardly hide the light source and “darken” a 5’ cube around it until discovered. Unless your room is a 5’ cube, that is.

 

[Mistwell]

Illusions cannot illuminate. An image of a shadow is not a real shadow.

Sage Advice (as quoted above) says if the illusion includes sensory effects and/or movement, then it casts a shadow.

Strong disagree.

Illusions cannot block anything (unless specifically noted). They have no physical reality. The notion that an illusion is enough present to block light but nothing else is special pleading.

Invisibility is an illusion spell, for what that is worth. Most of the mid to higher level illusions pretty clearly do block things. Most of the lower level ones pretty clearly do not.

This is all covered in that sage advice video linked to above.

 

[Storyteller Hero]

If anyone here would like to add even more variety to their use of illusions, I've written a book of over a hundred illusion spells for DMsGuild.

http://www.dmsguild.com/product/235311/THE-PRESTIGE-Spells-of-Illusion-5e

 

[Treantmonklvl20]

My general rule of thumb us that an illusion can add to a scene, but cannot subtract ir otherwise alter it.

This seems like a very good rule of thumb.