How did the Zalkonians suffocate the entire Enterprise crew?

Question

In Star Trek: The Next Generation, "Transfigurations," the Enterprise was intercepted by "Sunad," a Zalkonian tracking down "John Doe," the mutating Zalkonian. When Picard questioned Sudan's motives even after Sunad demanded John be turned over, he is seen on the viewscreen giving one of his crew members a look. Soon, the entire crew of the Enterprise, except for John Doe, starts suffocating.

Wikipedia's summary says this:

Captain Picard considers the situation, but refuses to release John to the Zalkonians without more information as to the charges. He contacts the other ship and mentions John's strange powers, which alarms Captain Sunad. Sunad immediately triggers a device which causes the entire Enterprise crew to become unable to breathe. John resists the device and heals everyone aboard the Enterprise with one bright flash of his energy.

So here, it's "a device," but how is there any mention of this later on? What is this device or system?

Answer 1

Main canon.

This device is never mentioned again in any of the shows or films and neither are the Zalkonians. For all intents and purposes, their existence is contained within this one episode, in much the same way that we never hear about the Douwd again.

The original screenplay doesn't offer anything more than what we see on screen;

Sunad nods to a subordinate offscreen. Before Picard can respond, he finds himself unable to breathe -- it's as if an invisible hand has him by the windpipe.

Everyone on the bridge is affected by the throat constriction. Worf manages to gasp out

WORF: Shields ineffective...

EU books/comics, etc.

The Zalkonians receive a fleeting mention in a couple of books, but only in the context of "species who've become pure energy" and never any mention of their weapon or ships.