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The Rogue archetype Arcane Trickster gains the feature Magical Ambush at level 9, which says

if you are hidden from a creature when you cast a spell on it, the creature has disadvantage on any saving throw it makes against the spell this turn.

Spellcasting > Targets explains:

A typical spell requires you to pick one or more targets to be affected by the spell’s magic. A spell’s description tells you whether the spell targets creatures, Objects, or a point of origin for an area of effect

The Area of Effect section adds:

Spells such as Burning Hands and Cone of Cold cover an area, allowing them to affect multiple creatures at once.

These wordings don't make it clear to me whether creatures affected by Area of Effect spells are also considered as having these casted upon them, and as a result the following question emerges:

Does Magical Ambush affect AoE spells?

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Magical Ambush works on any spells that force a saving throw (including AOE)

Jeremy Crawford has clarified this directly:

Q*: Does Magical Ambush impose disadvantage on AoE/line spells? Or is it only spells that target creatures?

A: Magical Ambush works with any spell that forces a creature to make a saving throw.

*Question was paraphrased for clarity

Which makes sense given he has clarified the topic of AOE targeting more generally:

A typical area of effect has more than one target: the effect's point of origin and one or more creatures/objects.

Given these two rulings, is reasonable to read "when you cast a spell on it" to actually be saying "when you target a creature with a spell" or "when you affect a creature with a spell".

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    \$\begingroup\$ Awesome, this is the kind of ruling I was failing to find. Thanks for the definitive answer! \$\endgroup\$ – hyperfekt Feb 27 '18 at 8:39
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Spells with Saves

You've quoted the entire text of Magical Ambush:

Starting at 9th level, if you are hidden from a creature when you cast a spell on it, the creature has disadvantage on any saving throw it makes against the spell this turn.

At multiple times, the designers of 5E have indicated that the game is meant to be parsed as normal language whenever possible.1 In other words, logic says if a spell affects a creature, then that spell was cast on that creature. Conversely, logic says if the spell wasn't cast on the creature, then they couldn't be affected by it. The fact that an area of effect is aimed at a point for targeting purposes, doesn't mean the spell is cast on that point alone.

It's pretty cut and dry - if the caster is hidden when the spell is cast, the target has disadvantage on the save. There's no more complexity to it. The type of spell doesn't matter, other than needing to be a spell that requires a save.

Spells with Attack Rolls

As an aside, if the caster is hidden and therefore unseen, he gets advantage on a spell attack roll, too.

1As @Slagmoth indicates in the comments, there is a DragonTalk podcast from Jan 19, 2017 where this is detailed by Jeremy Crawford, specifically addressing spellcasting.

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  • \$\begingroup\$ Is there any source for this? My understanding is that is principally possible that these spells are not considered cast on a creature but instead on an area with effect on creatures instead. \$\endgroup\$ – hyperfekt Feb 20 '18 at 19:34
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    \$\begingroup\$ Well, its answer is a necessary prerequisite to answer the question I asked in the absence of any other rules. \$\endgroup\$ – hyperfekt Feb 20 '18 at 19:53
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    \$\begingroup\$ @hyperfekt That's not how things work here. You have to explicitly ask the question you want answered. You could certainly include your confusion on that fact as part of the explanation for why you're asking this question. \$\endgroup\$ – T.J.L. Feb 20 '18 at 19:57
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    \$\begingroup\$ @hyperfekt Check out the Jan 19, 2017 Podcast for DragonTalk on the Wizards site. Crawford talks a lot about targeting and what it actually means. In short, yes, creatures within an area effect are considered targets by definition. \$\endgroup\$ – Slagmoth Feb 20 '18 at 20:10
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    \$\begingroup\$ @hyperfekt: here is a direct link to the Jan 19, 2017 Podcast of DragonTalk where they talk about targeting. \$\endgroup\$ – Rubiksmoose Feb 20 '18 at 21:37
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No, it does not affect the saves from area-of-effect spells. An AoE spell is cast on an area, not on a creature, so the "when you cast a spell on it" part is what determines that the Magical Ambush does not apply to this situation.

Magical Ambush could potentially apply to multiple targets (check with your DM, he/she may differ), so I would say that if, for example, the Rogue casts Hold Person using a higher level spell slot to cast it on more than one person, that still counts as being cast "on it", and the disadvantage would apply. Indeed, if the Rogue was hidden from some of the targets and not others, the disadvantage on the save would apply to each target he was hidden from, and not to any targets he was not hidden from.

But all of that is because in this second case, the spell is being cast on the creature(s), while in the first case the spell is being cast on a place and happens to affect anyone within the area.

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    \$\begingroup\$ This is pretty strongly contradicted by the extensive discussion from the Sage Advice segment in the official DragonTalk podcast from January 19th of last year. Jeremy Crawford goes into a great deal of detail (mostly in reference to Twin Spell, but it applies here) to say that the "targets" of a spell include anyone that is affected in any way by the spell. When you cast Greenflame Blade and hit one guy with your sword and a blast of flame does automatic damage to a second guy, the spell targets two creatures, and everyone in a fireball is a target of the fireball. \$\endgroup\$ – Darth Pseudonym Feb 20 '18 at 22:28
  • \$\begingroup\$ But being "the target" is not the same as having the spell "cast on you". And this is a good example of the distinction. Is the spell magic affecting creatures, objects, or area? \$\endgroup\$ – Phil Boncer Feb 20 '18 at 22:40
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    \$\begingroup\$ @PhilBoncer I understand what you are saying but I really think that in some instances we read way too much into how things are worded. Besides this does not explicitly indicate it has to be a single target. If something affected multiple creatures like an up-casted Hold Person you can easily say that you cast the spell on it, it and it. \$\endgroup\$ – Slagmoth Feb 21 '18 at 0:23
  • \$\begingroup\$ But "casting a spell on it" is a valid subset of "casting a spell on it, it, and it". Whereas "casting a spell on it" is not a valid subset of "casting a spell on a place near it". \$\endgroup\$ – Phil Boncer Feb 21 '18 at 2:55
  • \$\begingroup\$ The thing here is that I am NOT reading things into things, or relying on picky wording. I'm basing my position on how magic is consistently stated to work. Spells are cast on creatures, objects, or locations, and it does matter which for a wide variety of reasons. The Rogue's Magical Ambush is pretty clearly about spells cast on creatures, whereas any area of effect spell is, by definition, cast on an area. That's a difference with regard to magic, and one that makes a distinction here. \$\endgroup\$ – Phil Boncer Feb 21 '18 at 2:55

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