B is for Borshin:Zelazny A to Z

The Borshin is the secondary antagonist from Jack of Shadows and the subject of some of the best lines. Less a character and more of a force, it still moves the story forward.

It looked like something that had started out to be a man but had never quite made it. It had been stepped on, twisted, had holes poked into the sickly dough of its head-bulge. Bones showed through the transparent flesh of its torso and its short legs were thick as trees, terminating in disk-shaped pads from which dozens of long toes hung like roots or worms. Its arms were longer than its entire body. It was a crushed slug, a thing that had been frozen and thawed before it was fully baked.
It was-"It is the Borshin," said the Lord of Bats.

Later:

"I am Jack of Shadows!" he cried out. "Lord of Shadow Guard! I am Shadowjack, the thief who walks in silence and in shadows! I was beheaded in Igles and rose again from the Dung Pits of Glyve. I drank the blood of a vampire and ate a stone. I am the breaker of the Compact. I am he who forged a name in the Red Book of Ells. I am the prisoner in the jewel. I duped the Lord of High Dudgeon once, and I will return for vengeance upon him. I am the enemy of my enemies. Come take me, filth, if you love the Lord of Bats or despise me, for I have named myself Jack of Shadows!"
Quilian's face showed puzzlement at this outburst, and though he opened his mouth and tried to speak, his words were drowned out by the other's cries.
Then the window shattered, the candle died, and the Borshin sprang into the room.

and near the end:

Through the dust, the noise, the chill, it followed the trail. The flaring lights, the trembling land, the stalking storm meant nothing to it, for it had never known fear. It glided down hills like a ghost and slithered among rocks like a reptile. It leaped chasms, dodged falling stones, was singed once by lightning. It was a blob of protoplasm on a stick; it was a scarred hulk, and there was no real reason why it should be living and moving about. But perhaps it did not truly live-at least, not as other creatures, even dark-side creatures, lived. It had no name, only an appellation. Its mentality, presumably, was not great. It was a bundle of instincts and reflexes, some of them innate. It was lacking in emotions, save for one. It was incredibly strong, and capable of enduring extreme privation, great amounts of pain and excessive bodily damage. It spoke no language, and all creatures it encountered fled from it.
While the ground shook and the rocks rattled about it, it began its descent of the mountain-which-once-had-moved, currents of blazing cloud dropping fires along its way.
The landslide did not stop it any more than the tempest could.
It picked its way among the strewn boulders at the mountain's base and for a moment regarded the final ascent.
There led the trail; there must it follow.
High, high-set, walled and well guarded ...
But in addition to its strength it possessed a certain cunning.
...And its one emotion.

The Borshin even gets a song in the Jack of Shadows album.