Not a serious answer (unless it happens to be correct, in which case it is totally serious):
A saving throw
No matter the road they tread,
at the beginning I lurk.
The threat of the blown save can happen AT ANY TIME. Just having to make the roll means your face might be about to be melted off.
In darkness it is me they dread.
Entering that cave/dungeon, knowing there's gonna be a trap in there SOMEWHERE. Seriously, do dungeon architects take classes in this stuff?
I stalk and plot my strike,
eager to finish my work.
Seriously, that d20 hates you. It's just biding its time, waiting for the right opportunity.
It is after their long hike -
in the middle of their journey, my friend -
when I deliver my dirk.
Bloody random encounters.
Thus I finally bring the adventurer’s end.
Ugh, a 1. The owlbear does what to my what now?