A dark side ritual was an old Sith magical art that allowed a Force user to see things one could otherwise not see. It required a connection between two beings and a drop of blood from one of the two. The ritual was started by chanting a certain phrase and feeding the drop of blood with Force lightning on an altar. The blood donor then would have to focus on the other person.
The dark side ritual was a means by which Sith Lords could attack their enemies by wielding the power of dark illusion. To the intended victim, the illusion would appear to be true events and be unrecognizable as being an illusion.
Within the illusions, the person would be able to communicate with fictitious doubles of individuals they knew and be able to perform all abilities they could in reality. They would also be susceptible to pain and suffering within the illusions. The individual could also be killed within the illusion and as a result die in reality.
The ritual required the participation of two Sith Lords, one who shared a Force-bond with another Force user and a Sith Master to perform the actual ceremony. The two Sith would stand besides a sacrificial altar basin. The Sith who shared the Force bond would allow himself to be cut so a single drop of his blood could be added to the water in the altar basin. Once added, the water would turn black.
The ceremony would then begin as the Sith master started to recite an ancient Sith incantation in the Balc tongue. Near end of the incantation, both Sith would strike the water with Force lightning which caused it to start to emit an eerie mist. To enact the illusion, the Sith Master would then immerse his face towards the basin and receive the ability to cast the power of dark illusion and enter the mind of their victim to manipulate it.
If the intended victim was a Jedi, the Sith could mask their presence by coming to the Jedi as a familiar face of someone they trusted.
Darth Sidious conducts the Dark side ritual.
The ritual was used with great effect by Sith Lord Darth Sidious. At some point near the end of the Clone Wars, Jedi Master Yoda concluded a journey to discover the technique of spiritual preservation after death by traveling to the ancient Sith World of Moraband. Darth Sidious sensed his presence there, and summoned his apprentice Darth Tyranus to the LiMerge Building in order to attack Yoda with the dark side ritual. Through the ritual, the two Sith Lords were able to create an illusion which overpowered Yoda's mind inside Darth Bane's tomb.
Sidious first came to Yoda in the guise of long lost Jedi Master Sifo-Dyas and tempted him with the knowledge of Sidious' identity. Sidious then plunged Yoda into an illusion where he and fellow Jedi Knight Anakin Skywalker were closing in on the Siths' secret lair. During a fierce lightsaber duel, Sidious taunted Yoda with the fact if he allowed Anakin to die, he would be able to stop Sidious and prevent his attack on the Jedi Order yet the Jedi Master allowed himself to be sacrifice in order to save Anakin. Yoda, however, survived the ritual with his mind intact.