Nahel bond
The Nahel bond is the spiritual bond formed between a human and a spren that grants the human the powers of a Knight Radiant in Brandon Sanderson's The Stormlight Archive. It is the source of the Surgebinding abilities wielded by the orders of Radiants.
Nature
The Nahel bond links a person to a sapient spren, drawing on the intent of Honor and the bonding nature of Roshar. Through the bond, the human gains the ability to draw in Stormlight and to manipulate two of the fundamental Surges, the forces that govern Surgebinding. In return, the spren gains greater awareness and a more solid presence in the Physical Realm; without the bond, such spren remain primarily creatures of the Cognitive Realm.
Oaths and Ideals
The Nahel bond deepens in stages as the human speaks the Ideals of their order, sworn oaths that bind them to the principles of the Knights Radiant. The First Ideal, "Life before death, strength before weakness, journey before destination," is shared by all orders, while each order has its own subsequent Ideals. As the bond strengthens, the Radiant gains greater access to Stormlight, the use of Shardblades formed from the spren itself, and other abilities.
Breaking the bond
The bond can be broken if the Radiant violates the oaths or rejects the relationship. In the ancient past, the Knights Radiant abandoned their oaths in an event called the Recreance, severing their bonds en masse and killing their spren, leaving those spren as "deadeyes." The spren of Roshar long remembered this betrayal, making them wary of forming new bonds.