Many spells that inflict AOE damage over the course of more than one turn describe that the subject takes the damage when it enters the area or starts its turn there.

I quote for example the PHB spell cloud of daggers:

A creature takes 4d4 slashing damage when it enters the spell's area for the first time on a turn or starts its turn there. (PHB, p. 222)

It seems pretty clear, but I had some doubts in a peculiar situation. We were fighting some underground worms and our party's sorceress readied the cloud of daggers spell to target the first enemy that would have appeared. A worm jumped out of the ground and she cast the spell, but she fell unconscious right away (the worm attacked her) and so the monster didn't actually start any turn in the area nor moved in, so the master negated the damage.

Rationally it makes little sense to me that for one turn it stayed in the cloud and still didn't take damage. Did I misunderstand how the events took place?

My questions are:

  1. Do you know a rule/official tweet that explains more deeply this kind of situation?
  2. How could the situation be described so that it would have made sense?
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    \$\begingroup\$ If you are asking about Cloud of Daggers, you should limit your question to that. Each spell has different mechanics and to ask for all AOE is too broad. \$\endgroup\$ – NautArch Aug 27 '18 at 18:56
  • \$\begingroup\$ Question 2 could be amended to something like "Is there any way the character could have used a readied cloud of daggers spell to ensure the worm takes damage in this situation?" That would be more objectively answerable as opposed to opinion-based. \$\endgroup\$ – Ryan Thompson Aug 27 '18 at 19:02
  • \$\begingroup\$ I modified your question in a few key ways. 1) I made it focus on the one spell you cared about Cloud of Daggers. Focusing on all AOE spells was going to be too broad. 2) I edited your second question to not be an opinion/poll question (which we cannot answer here). Please edit to revert or improve further if there are things you disagree with. \$\endgroup\$ – Rubiksmoose Aug 27 '18 at 19:07
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    \$\begingroup\$ Thanks for the corrections. But personally I don't think that asking about aoe spell would be too broad: I think it's clear that I'm considering all those spell that have the formula ",When a creature enters the spell's area for the first time on a turn or starts its turn there" and they are many (web, grease, gust of wind, moonbeam, sleet storm, Evard's Black tentacles, cloudkill are just some examples) \$\endgroup\$ – YED - Yellow Endless Dog Aug 27 '18 at 19:31
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    \$\begingroup\$ @YED-YellowEndlessDog: Why do you want to ask about so many different spells when your entire question revolves around a scenario involving just one spell? And if what you say is true that means that every answer should work just fine with all those spells regardless of whether you ask for them or not, so why complicate your true issue? On this site, it is better to focus on the core of your problem and answers can and often do go broader than they need to to answer it. \$\endgroup\$ – Rubiksmoose Aug 27 '18 at 19:38

Your understanding is correct

To reiterate what happened:

  1. Sorceress readies cloud of daggers to cast on the first enemy that appears.
  2. Worm appears.
  3. Sorceress casts cloud of daggers on top of the worm.
  4. Worm attacks Sorceress.
  5. Sorceress loses consciousness and spell vanishes.

The only two times a creature takes damage from cloud of daggers is:

A creature takes 4d4 slashing damage when it enters the spell's area for the first time on a turn or starts its turn there. (PHB 222)

And you are correct in saying that neither happened in this case.

But, in a Sage Advice article, it is officially clarified:

Our design intent for such spells is this: a creature enters the area of effect when the creature passes into it. Creating the area of effect on the creature or moving it onto the creature doesn't count. If the creature is still in the area at the start of its turn, it is subjected to the area’s effect. [...]

In summary, a spell like moonbeam affects a creature when the creature passes into the spell’s area of effect and when the creature starts its turn there. You’re essentially creating a hazard on the battlefield.

The worm could have taken damage if the Sorceress had cast it differently

If the Sorceress had cast cloud of dagger ahead of the worm instead of on top of it, then it would have taken damage for entering the spell's area. Of course (as @NautArch pointed out), if they cast it this way, the worm could either move around it or choose to not go through it at all. But even if it chooses this, the AOE has served some battlefield control purpose and likely kept the Sorceress conscious. So still a possible win.

How to make it make sense

This was probably just a case of the player not thinking the exact wording of the spell through enough and expecting it to work a different way. In cases like these, I as DM would have simply allowed the player to place the cloud 5 feet further from the monster such that is enters the area and takes damage. So that would have been one way to fix this issue.

Another would be to narratively describe it in a way that makes sense. For example, "The worm charged through the cloud of daggers just as the daggers were still forming, taking no damage and knocking Sorceress out before the daggers could pierce its thick hide."

  • \$\begingroup\$ I've deleted my answer as I didn't realize he had the spell readied and instead assumed he had already cast it. Your interpretation seems correct. \$\endgroup\$ – blurry Aug 27 '18 at 19:38
  • \$\begingroup\$ I like your answer. I didn't really think about casting it ahead of the worm and it's a good solution. As a DM I would also have explained it as you said: the cloud started to form but vanished too fast to deal any damage to the worm \$\endgroup\$ – YED - Yellow Endless Dog Aug 27 '18 at 20:31
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    \$\begingroup\$ @YED-YellowEndlessDog Buyer beware, though as Rubiksmoose has said: the worm would likely have just avoided the 5' cube and if it had enough movement it would resulted in the same events. \$\endgroup\$ – NautArch Aug 27 '18 at 20:49
  • \$\begingroup\$ I didn't said it because I wanted to be brief, but in fact we were in a tunnel 15 foot large and the worm was large. Unless it digged under the cloud, it couldn't have just avoided it. \$\endgroup\$ – YED - Yellow Endless Dog Aug 27 '18 at 20:55

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