The Black Guardian is a character in the long-running British science fiction television series Doctor Who, portrayed by Valentine Dyall. He is an anthropomorphic personification of the forces of entropy and chaos, the counterpart of the White Guardian, a personification of order. The two Guardians balance out the forces in the universe, although the Black Guardian seems to desire to upset the balance in favour of chaos and evil while the White Guardian prefers to maintain the status quo.
The Fourth Doctor 
- Portrayed by Tom Baker
The Armageddon Factor [16.6] 
- Black Guardian: I expected no less of you! You whimpering wraith! But your death is already compassed in my designs. For now, the Doctor shall release the key to ME, and chaos shall break upon the universe!
Logopolis [18.7] 
- Black Guardian: DOCTOR! YOU SHALL DIE FOR THIS!
The Fifth Doctor 
- Portrayed by Peter Davison
Mawdryn Undead [20.3] 
- Black Guardian: (to Turlough) I am your guardian.
Terminus [20.4] 
- Black Guardian: (to Turlough) You have work to do.
- Turlough: What does this do?
- Black Guardian: You are touching the heart of the TARDIS. Rip it free!
- Black Guardian: (to Turlough) This is your last chance boy! KILL THE DOCTOR!