Does Magical Ambush affect Area of Effect spells?

Question

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?

Answer 1

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".

Answer 2

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.

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

Answer 3

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.