The Force-bond ability became prominent in Star Wars: The Last Jedi, where Rey and Kylo Ren begin to use the power to communicate with each other. This is not the first time in the Star Wars realm that a Force-bond had been used. In fact, it has been used for many years prior to the events of The Last Jedi by both Sith and Jedi alike. Although it has been used by both the Light and Dark Side, it is considered a Dark Side practice due to how dangerous it is.
By definition, a Force-bond is a powerful connection between two Force-sensitive individuals. It allows them to communicate telepathically, visually, and emotionally. The bond can range across great distances, allowing the individuals to remain psychically linked, even if on opposite sides of the galaxy. When the Force-bond is in action, both parties manifest in each other's location at the same time, allowing for them to interact, almost as if in person. In rare instances, the individuals can be capable of physical contact, but this is often only the case for Force dyads. Occasionally, the bond is even capable of teleporting matter.
However intriguing and powerful the Force-bond may seem, it can potentially come with consequences. The 2019 book Star Wars: The Secrets of the Jedi lists it under "Dark Force Abilities."
"The Force allows us to connect with another living being and communicate with them across great distances, seeing what they see and feeling what they feel," the book says. "Though this may seem like a harmless - and perhaps even valuable - ability, it is easily manipulated by those on the Dark Side. Some powerful Force users have been able to create secret bonds with others who are unaware of their connection. They then use those bonds to corrupt their target and steer their actions. Even if the unwanted bond is detected, it can still be extremely difficult to break."
For example, a Sith could use the bond to manipulate the Force visions of their target through the use of another Force power known as a dark illusion, which is an ancient Sith practice. It requires the Sith to collect a blood sample of their target, throw that in a sacrificial altar basin and add a flash of Force lightning. The Sith would then chant an old Sith incantation in the Balc language. At this point, the ritual would be complete, and the user would now be able to peer into the bonded target's mind.
This dark illusion method was used by Count Dooku and Darth Sidious during the Clone Wars. Before his turn to the Dark Side, Count Dooku was a Jedi Master, trained under his mentor, Master Yoda, developing a strong Force-bond with Yoda. He later fell to the Dark Side, but the bond remained intact.
During the Clone Wars, Sidious realized he could use Dooku's Force-bond against Yoda. He summoned Dooku to Coruscant, where they performed the ancient Sith ritual, giving them the power of dark illusion. Sidious then used the dark illusion to attempt to lure Yoda to the dark side of the Force, however, Yoda resisted. This resulted in a battle of the wills between the Jedi Master and Sith Lord, with the Sith being unable to break the Jedi.
Another way to create a Force-bond is through the fusion of two holocrons. A Light Side wielder and a Dark Side wielder would have to work together to combine a Jedi holocron with a Sith holocron. The result would be a psychic connection between the two force users.
This Force-bond was created between Jedi Padawan Ezra Bridger and former Sith Apprentice Darth Maul. During the height of the Galactic Empire, Darth Maul was desperate to find Obi-Wan Kenobi to get revenge against him. Thinking that Ezra could help him in the search, he captured the Padawan's allies, using them as leverage to convince Ezra to create a Force-bond. Ezra complied, and the two worked together to fuse the holocrons, thus establishing a psychic link between them. After their encounter, Ezra experienced a series of hallucinations and visions relating to Darth Maul, causing great psychological distress to the Jedi.
The most prominent example of the Force-bond was shown between Rey and Kylo Ren, who were a unique case. They were a prophesied dyad in the Force, the likes of which would prove to be the most powerful Force-bond yet. They dyad discovered their bond after the destruction of Starkiller Base. The mysterious connection bridged their minds, allowing them to see and hear each other across millions of light-years. As Rey's powers increased, so did their Force-bond.
Unlike the other Force-bond relationships, Rey and Kylo's is different. At first, Rey finds the bond alarming and irritating, for she does not want to communicate with Kylo. However, as they begin to communicate through the bond more, both parties become increasingly connected.
Instead of using their Force-bond for manipulation, they listen to each other and confide in one another. Eventually, their connection grows so strong that they are able to touch each other's hands through the Force-bond. At this touch, they experienced a shared premonition of the future. This instance convinced Rey that she could turn Kylo to the Light Side, and as their bond grew stronger, it allowed them to transfer objects to each other from great distances.
The ability to create a Force-bond is powerful, exciting, and desirable, but it is proven to be dangerous and easily manipulated. By looking into the definition of the Force-bond, the ways in which it is usually produced and the previous methods of using it, it is reasonable to believe that it is inherently a Dark Side practice.