Likely answer

Fantasy Books Featuring Assassins and Symbiotic Magic

The book you are looking for is "The Way of Shadows" by Brent Weeks. It is the first novel in the Night Angel trilogy and follows Azoth, an street urchin who becomes an apprentice to the ultimate wetboy, Durzo Blint. The story features a powerful, sentient magical artifact known as a ka'kari—a rare black orb that bonds with the protagonist, granting him symbiotic abilities and immense power.

Confirmed match: The Way of Shadows by Brent Weeks