The Nahel bond is a connection between a Spren and a Rosharan human. This bond is a symbiotic relationship, granting the human participant the power of Surgebinding and the spren participant the ability to manifest into the Physical Realm.
Originally, the Nahel bond granted power to individuals with no affiliation with one another. Eventually, the Knights Radiant were formed and the Surgebinders were gathered into ten different orders of Knights, each order's members bonded to the same type of spren.
Formation of the Bond
Spren seem to bond with people who share in their perspective or personality. However, some types of spren are more discerning than others when choosing their human partners. Additionally, some types of spren are more free-spirited and choose the humans that they bond with individually. Others are more organized and will choose a human to bond with by committee, then send one of their members to the chosen human.
Limitations of the Bond
Spren bond with people that share beliefs similar to the Spren's own Concept. As the Bond progresses, the holder of the Bond will move along a path bringing the holder's thinking more in line with the Spren they are bonded to. Each Ideal said by the Radiant progresses the bond. Only the first Ideal is said by all Radiants, as the later ideals are Order specific. Lightweavers say Truths rather than Ideals. Without saying the words, the Radiant's ability to use Stormlight will not increase. If the Radiant stops acting according to their oaths, they will weaken or completely destroy the bond, which can result in killing the Spren.
Powers granted by the Bond
The Nahel Bond grants the human the ability to Surgebind. Surgebinding allows one to use Stormlight, which grants superhuman abilities to the user, such as increased strength, agility, and healing abilities. It also perfects innate skills of the user but does not grant additional skills; for example, an expert spearman will become a spearman without equal, but will not become a master swordsman. In addition, Stormlight fuels the primary magic of Surgebinding: the usage of Surges.
Each type of spren will grant its human Surgebinder two different Surges. Which two is decided by the type of spren to which they are bonded, and thus, which order of knights they belong to. There are ten different types of surges, and ten different orders of knights, so each surge is used by exactly two different orders of knights. There is a cognitive connection between all people who are bonded with a certain type of spren. This mental connection provides telepathy and recognition between the Bond members. Some Orders of the Knights Radiants have additional, unique abilities. For example, Lightweavers are known for their mnemonic abilities (which manifests differently in each person) and Skybreakers can divide the innocent from the guilty. This is a result of the two surges overlapping, producing an effect like mixed chemicals.
When the Nahel bond is at a sufficient strength, the bonded spren can instantly manifest itself as a Shardblade in any number of forms. For example, when Kaladin and Syl are in battle, Syl is able to manifest herself as a sword, a spear, and a shield. A human using a Shardblade without a Nahel Bond (these shardblades are dead spren) must wait ten heartbeats before the summoning will take place, and are limited to using the Shardblade in the form of a large sword, as Shardblades were patterned after the original Honorblades.
The Nahel Bond also grants the ability for spren to manifest themselves into the Physical Realm, outside of the form of a Shardblade. Additionally, the spren gains greater self-awareness, intelligence, and access to their memories the stronger their bond is with their human.
Severing the Bond
The only known way to sever a Nahel Bond is for a Bonded Person to act in a manner contrary to the limitations of the Bond. This can kill the spren, sending them back to the Cognitive Realm. Revival is possible, but difficult. It is even more difficult for the spren to bond with a different person after severing the bond, and they will only do this if the person they originally bonded with is dead. It is uncertain if a Spren is able to sever the bond on their own and return to the cognitive realm.
Notable Surgebinders and Spren
Alakavish was a Surgebinder and yet the Nahel bond gave him no more wisdom than a regular man”
”The responsibility of what we've been given — whether it be the crown or the Nahel bond — needs to make us better
- The Way of Kings chapter 60
- How do spren choose what human they'll bond with?
— Theoryland - Dec 6th, 2014