How can Silent Image be used to obscure vision in combat in 5E?

Question

In my party I have a warlock who is partial to illusion magic. At second level, he took the 'Misty Visions' invocation, with the intent of using Silent Image to keep the party shrouded in illusory fog during combat. His logic is that because the party knows it is an illusion, they are never hindered by it, but the enemies are effectively blinded to the players until they manage to discern the illusion. In one instance, this resulted in two party members swinging with advantage on a bugbear who couldn't see them, and who had to use his action on the next turn to inspect the illusion.

In this situation, Silent Image gives most of the benefits of Darkness, a second level spell, without negatively affecting the party, and without costing a spell slot. I don't want to have to tell the player "No, that's too OP," but I'm not sure how to interpret the mechanics in a way that isn't broken.

In short:
What does RAW say about using Silent Image on top of or between close quarters combatants?
How can Silent Image be interpreted/altered such that it can provide some benefit in combat, while maintaining the intended power level?

Answer 1

Cost

This is not without cost to the warlock; he has chosen to use one of his two invocations to get this thereby forgoing other choices. In addition, he uses an action to cast it and an action to move it; unless your battles are very static he would need to move it a lot. Remember, the most limited resource any creature has is not its spell slots or hit dice; it is its actions, it only gets one per turn. Good players know this and they should be thinking every turn "Is this the best thing I can do with my action right now?"

Innovation

This is a very clever and imaginative use of the spell - this is something that you should encourage in your players; not discourage. I have had a wizard use Silent Image to create an picture of a hallway that the rogue could walk behind; this not only gave advantage, it also allowed sneak attack against, coincidently, a bugbear.

Disadvantages

A 15 foot cube of fog rolling towards the bugbear is going to negate surprise (its just not natural) and allow it to make an active perception check to find out where the PCs are in the cloud. The bugbear can then use its action to attack (with disadvantage); following which it can see through the cloud because "Physical interaction with the image reveals it to be an illusion" - sticking your morning star into it qualifies as "physical interaction".

It doesn't work that way, anyhow

You say: " because the party knows it is an illusion, they are never hindered by it". Where does it say that in the rules?

The relevant text of the spell is:

Physical interaction with the image reveals it to be an illusion, because things can pass through it. A creature that uses its action to examine the image can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the illusion for what it is, the creature can see through the image.

There are only 2 ways to see through the image, "Physical interaction" or "use your action" and make a save. Knowing that it is an illusion doesn't allow you to see through it.

Answer 2

I agree with Dale M that you should reward innovation, but it sounds like the player is overlooking one of the aspects of the spell:

"You can use your action to cause the image to move to any spot within range. As the image changes location, you can alter its appearance so that its movements appear natural for the image."--PHB pg 276

This implies that the illusion is completely still when the caster is not using an action to move it. So, the fog wouldn't roll; it would be a hazy, frozen, very obviously unreal phenomenon...so you'd be justified in saying that it wouldn't work on most enemies because they'd be able to passively see how wrong it is.

Answer 3

I believe that it would stand to reason that physical interaction would still reveal the illusion, despite it being a fog. I mean this because, although fog is gaseous- and therefor allows things to pass through it normally-, a thick fog like the one described for this illusion can be felt; it normally feels like a mist brushing over you. This is slight, but it is a sensation that the spell would not replicate.

Answer 4

The PhB 254 states:

Physical interaction with the image reveals it to be an illusion, because things can pass through it. A creature that uses its action to examine the image can determine that it is an illusion with a successful Intelligence (Investigation) check against yo r spell save DC. If a creature discerns the illusion for what it is, the creature can see through the image.

He can see through the illusion

Regarding physical interaction. If the bugbear touches the fog (which is a lot of physical interaction for me) he would immediately be able to see through it. If he throws/shoots to someone something and the projectile passes through the illusion, he would immediatly be able to see through it.

If the bugbear see a lot of proyectiles and missiles coming from an odorless, heathless fog, he would probably run in it, touching the fog and therefor been able to see through it.

Answer 5

Reward them the first time. Allow it's use and don't always counter it, but be aware that it is a standard tactic and setup instances where it doesn't/can't work and/or would backfire.

• The enemy runs from a fog throwing arrows/spears/daggers/spells, causing the warlock to waste actions moving the fog in pursuit. Perhaps through dangerous terrain or traps that the enemy knows about that the players don't.

• Set up enemies that can "see" through it. Blindsight, etc. (as others suggested)

• Have some invisible enemies -- like a Faerie Dragons or Imps, so they are both at disadvantage.

• Have a room already full of poisonous smoke, so the players venture in thinking it is just the warlock's spell. (The spell allows you to "see through it" but you still see the illusion as well).

• Have a friendly NPC begin attacking the unseen thing bursting through the door. The then need to not hurt him, but calm him down until the Warlock can cancel the fog.

• Have an enemy use similar tactics on the party, (and see if they complain about fairness.) Fey are versed well in illusions.

• Depending on the Warlocks Patron, you could have the Patron not amused at how the warlock is using its gifts -- but that would be a last resort.