Death ward protects against two things:
* Death spells (i.e., spells with the Death descriptor)
* Magical death effects (i.e., magical effects with the Death descriptor)
Please point to the line of the spell that states only those effects with the [death] descriptor.
As it is that Symbol of Death (in its name) that blatently kills in all ways just like every other death spell, is also not a death spell.
Now considering that, and that no supernatural effects ever have tags....
not the aboleths Enslave, which can obviously effect those immune to mind effecting powers, as it doesn't state it is one
not the Allip's babble, as though it says 'mind affecting' it isn't in tags...
The Behir's breath weapon is not really lighting because it does not have the [electricity] tag....
or the Bodak's death gaze.... Which appraently, according to you, effects undead and constructs!
....
Since your defintion does not hold up to example, it must be flawed.
If your definition is flawed one must return to the description
As such that is what one does, where you could say 'death' spell mean those labled with only the tags, legitimately as SPELLS are given such tags, magical effects most defintely does NOT have this limitation. The books completely fail to use these tags at all.
The use of 'magical-effects' leads one to immediatly conclude that this indeed must have been added for a purpose. Effects like the Bodak's death gaze pretty much confirm it. In such one declares that 'magical-effects' must be a catagory of at least one entity. Given this and the fact that no supernatural effects, or effects other than spells are labled with the tags, one must presume that your definition is indeed wrong.
That is pure logic.
From a common sense perspective if it is a spell that 'kills' it is a death spell. Afterall that is the english definition of a death spell. Further if it is a effect that kills, of magical nature than it is a magical death effect.
Phantasmal killer kills with 'magical fear' as such it is a magical death effect. So would be a spell that kills by stopping the heart, a spell that kills by causing the body to explode, or whatever other method an individual dreams of.
If it kills (save or die) it is a magical death effect.
It is pretty simple.
If that is not enough, consider this. The fortude save is used only for the following:
#1 : Save against poision (which now always does ability damage)
#2 : Save against disease (which also does abiilty damage)
#3 : Save against bodily change, which is nicely marked by 'polymorph' or other traditional mythical translations.
#4 : Save against death.
Phantasmal Killer requires #4. If one is making a save against death, one presumes one is effected by a death effect. And Phantasmal Killer is magical.
Thus it is a magical death effect.