Rage of the Stormdancer by k-t-roo
"Slickwind galloped, searching for the white colt that he had been ordered to kill. It was dark and the rain lashed his eyes. He slid to a halt, slipping on the wet ground he blinked and scanned around. He heard a creature in front of him, the snorting angry breaths, and the stamp of hooves. A streak of lightning arched the sky and his heart clenched in terror as before him he saw a stallion as black as obsidian. A stallion he thought long dead, whose heart-mate he had taken, and whose red eyes glinted angrily with burning malice in the stormlight, and whose dark horn slit the air, sharp as a knife."