"Song of a Mad Minstrel" by Robert E. Howard. Allow me to share it with you (again):

I am the thorn in the foot, I am the blur in the sight;

I am the worm at the root, I am the thief in the night.

I am the rat in the wall, the leper that leers at the gate;

I am the ghost in the hall, herald of horror and hate.

I am the rust on the corn, I am the smut on the wheat,

Laughing man’s labor to scorn, weaving a web for his feet.

I am canker and mildew and blight, danger and death and decay;

The rot of the rain by night, the blast of the sun by day.

I warp and wither with drouth, I work in the swamp’s foul yeast;

I bring the black plague from the south and the leprosy in from the east.

I rend from the hemlock boughs wine steeped in the petals of dooms;

Where the fat black serpents drowse I gather the Upas blooms.

I have plumbed the northern ice for a spell like Frozen lead;

In lost grey fields of rice, I learned from Mongol dead.

Where a bleak black mountain stands I have looted grisly caves;

I have digged in the desert sands to plunder terrible graves.

Never the sun goes forth, never the moon glows red,

But out of the south or the north, I come with the slavering dead.

I come with hideous spells, black charms and ghastly tunes;

I have looted the hidden hells and plundered the lost black moons.

There was never a king or priest to cheer me by word or look,

There was never a man or beast in the blood-black ways I took.

There were crimson gulfs unplumbed, there were black wings over a sea;

There were pits where mad things drummed, and foaming blasphemy.

There were vast ungodly tombs where slimy monsters dreamed,

There were clouds like blood-drenched plumes where unborn demons screamed.

There were ages dead to Time, and lands lost out of Space;

There were adders in the slime, and a dim unholy Face.

Oh, the heart in my breast turned stone, and the brain froze in my skull-

But I won through, I alone, and I poured my chalice full

Of horrors and dooms and spells, black buds and bitter roots-

From the hells beneath the hells, I bring you my deathly fruits.