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I'd introduce the build up first. The appearance of a DEM is difficult to overcome with subsequent explanations, IMO without a hint of what is happening, this taints your story, especially if the ...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/35452 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/35452 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
### I'd introduce the build up first. The appearance of a DEM is difficult to overcome with subsequent explanations, IMO without a hint of what is happening, this taints your story, **especially** if the guy with power is not that surprised and seems to know they were capable of this all along (as will be the case if the power is shown to be sought, later). One exception is well-known comic-book heroes (or similarly well-known fictions, e.g. Transformers), so the reader knows what to expect from the start; we would not be surprised if a pre-Batman Bruce Wayne showed extraordinary inventiveness or fearlessness in battle. Another exception is **settings** that allow for constantly new things: Magical or SciFi. It does not seem a DEM if some character in Lord of the Rings shows a magical ability for remote viewing or starting a fire. It does not seem a DEM if a brand new Star Trek being has the ability to divide itself into multiple beings, or magically transport a space ship to another galaxy. But your setting does not sound like such a place, and your character is not already famously established with your audience, so I don't think these exceptions apply. I would show some small hint that this power is possible; some small manifestation, then it is plausible that under duress, injured, bleeding and about to be killed, your character can summon something a thousand or million times greater than what he was doing for fun. Instead of struggling to light a candle with his mind and only succeeding after several sputtering starts, in his desperation in battle he incinerates the villain as if in a roaring furnace. Or whatever your particular power is.