If a commoner dies from a phantasm dealing damage to them (thinking it was a blade that slashed through them) - what would be the cause of death?

I mean, if a detective is investigating a murder committed with a phantasmal force, would it seem like the victim had a stroke? Would there be any slashing marks at all? Or could they immediately discern it was a psychic attack?

I'm thinking of making a mystery adventure where the killer uses phantasmal force, so it's hard to track them. But is it impossible? Detect magic could probably show traces of illusion, but any other traces it could leave, or is that the perfect crime??

I love the creative concept. I'd say a good medicine check would see that the damage was perceived and therefore became real as we can't always associate physical results in a world of Magic to make sense to us above table. If you wanted a more "realistic" approach, it could be determined that it was a mental rupture of the victim, but only in the same lines that a placebo can heal in medical trials as the mind is a powerful tool

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It will heart attack because killed by psychic damage.

Realistically speaking, the cause of death would likely be due to shock; the target perceived something fatal, and as such, the brain and body reacted to it fatally since the victim believed it to be real, thus ultimately leading to the victim's demise. Honestly, the victim could be seen acting strange by witnesses before just dropping dead. Spells like Speak with Dead might reveal what the victim thinks they saw despite there being zero actual evidence of it and Detect Magic would likely reveal traces of Illusion magic on the victim since the spell was cast on them. Identify, too, would work similarly.

If you want to be REALLY evil, you can make the victim think they're having a heart attack or something similarly invisible like a stroke or an aneurysm. One moment, they're shopping around the local fruit vendor, and then all of a sudden, they grab their chest and drop dead. Such is the beauty of illusion magic, after all.

I'm a nutjob for murder mysteries; creating them and solving them. Hope I gave you plenty to work with.

No scaling....sigh

You can use this to recreate the effects of other 2nd-level spells like blindness/deafness by creating the illusion that the target has a splotch of ink on it's eyes or a irremovable blindfold.  Or create an illusion of swarming insects around it's head to cause blindness, deafness or at least disadvantage on Perception checks involving hearing (the noise of the insects would make it hard to hear things around it), and psychic damage over time (the insects biting or sting the creature). 

You could also make the creature believe it's trapped inside a dome of lava or whirling sawblades, but a gutsy creature can run through the wall (the wall does not stop creatures) and only take 2d8 psychic damage escaping.  

Quote from Negelum >>

No scaling....sigh

 Cause it isn't a damage spell.  It does damage, but that's not it's primary purpose.  It's an illusion spell that's feels extra realistic to the victims.  

I wouldn’t rule it that way. Spells should be distinct and Phantasmal Force shouldn’t be able to cause conditions because the description doesn’t say that. If you want to blind then you use the Blindness spell. The description says the “target treats the phantasm as if it were real and rationalizes any illogical outcomes from interacting with it.” So the target may try to remove the blindfold or ink but it wouldn’t be able to and it rationalizes that it’s too tight or difficult to remove. But it would not actually prevent it from using its senses. It would rationalize that there must be gaps in the blindfold or that despite the ink it could still see. Or that the sound of the insects weren’t enough for it to stop hearing others because its hearing is really good. The spells adds something to the target’s reality but it doesn’t diminish its senses.