It is invisible to the eye,
yet it is a stranger to no one.
It weighs less than air,
yet it is sharper and more keen than any knife, sword, or blade.
It comes from nothing, but it has the power to vanquish armies.
What is it that I am speaking of?
The answer that I remember was:
hunger
but it could also be
time or cold.
You are speaking of
Time
It is invisible to the eye
You cannot see time
yet it is a stranger to no one
But everyone is familiar with it
It weighs less than air
It has no weight
yet it is sharper and more keen than any knife, sword, or blade.
Time cannot be blocked, it pierces any armour.
It comes from nothing
Time has no 'source' of which we know
but it has the power to vanquish armies.
The passage of time can destroy anything, including armies
The answer is:
Intellect
It is invisible to the eye,
Intellect is abstract so it can't be seen.
yet it is a stranger to no one.
Everyone has intellect (ranging from a little bit to a lot).
It weighs less than air,
Intellect is abstract so it has no weight.
yet it is sharper and more keen than any knife, sword, or blade.
Some people are said to possess a sharp or keen intellect.
It comes from nothing,
Intellect comes from the mind, which due to its abstractness could be considered nothing.
but it has the power to vanquish armies.
One must typically use intellect to win a war.
The answer is
Thought
It is invisible to the eye
You cannot see thought.
yet it is a stranger to no one
Even a strange thought is not a stranger
it weighs less than air
of course
it is sharper than a keen knife, word or blade
thought can pierce the vastness of space, or the microcosm of an atom which a blade cannot do
it comes from nothing
who can truly say where a thought comes from? some theories speculate that even thoughts we consider our own, might originate from the unknown (nothing)
but it has the power to vanquish armies
indeed, without thought, one cannot vanquish an army.