Darth Vader

Lord Vader, also known as Darth Vader by his secret Sith name, was Darth Sidious' second-in-command and due to this and his many titles rules the Galactic Empire in the Emperor's name. He is the Empire's main leader, a brilliant strategist and a skilled warrior who hunts down the rebel cell leaders and members of Rebel High Command like Mon Mothma.

Sending a messenger and calling Krennic
After the Death Star is finished, Vader and Sidious send Governor Wilhuff Tarkin to learn about the project's last phases and to test out the battle station's destructive weaponry. After the test on NiJedha city, Tarkin assumes command of the Death Star and tells Death Star director Orson Callan Krennic that the defector, Bodhi Rook, that had been in the city and had attempted to warn the rebels of the Death Star (which unknown to them had succeeded) had been sent by the traitorous Galen Walton Erso from Eadu. After the mission to Eadu, Krennic is told by Captain Dunstig Pterro that Lord Vader had called them to his castle on Mustafar.

Visit on Mustafar
Krennic arrives in his shuttle at Vader's castle soon after he had been called there. Vaneé and tells Vader of Krennic's arrival as Vader prepares to leave his bacta tank. Vader and Krennic discuss the Death Star and near the end he tells the director to ensure that Erso's message to the rebels will not cause an attack by the rebellion that might compromise the weapon. When Krennic foolishly asks Vader if he is still in command of the Death Star, Vader begins Force-choking him and tells him to beware not to let his aspirations cloud his better judgement.

End of the Battle of Scarif
As the end of the battle of Scarif is near, Tarkin asks the Imperial Lord to come and stop the rebel ships from retreating. At the battle's end, Vader's Imperial I class Star Destroyer, the Devastator, arrives and opens fire upon the rebel space crafts. After Captain Shaef Corssin tells him that the rebel flagship known as the Profundity has been disabled, but has received a transmission from the planet's surface, with the stolen Death Star plans, Vader takes a few Stormtroopers and boards the ship. As he enters a room, Vader proceeds to brutally cut down and choke surrounding Rebel Troopers in his attempt to reach the plans, but ultimately a few rebels escape with the plans to Princess Leia Organa aboard the Tantive IV.

Deleted Scenes
In a deleted scene, shortly before Krennic meets with Tarkin, Tarkin discusses with Vader about Krennic compromising the Death Star and that he should be removed from the project. Later when Tarkin assumes control of the Death Star, Krennic raises his blaster pistol to fire at the Moff only for his weapon to fly from his hand. Krennic and the others look surprised as the pistol is in the hand of Vader, who had unexpectedly arrived. Vader orders everyone except Krennic to leave and asks the man why had he threatened one of his emissaries. Krennic tells Vader that the Death Star is his work not Tarkin's and goes on to explain it's power.