Life force was basically the "soul" of a living being. (TV: The Doctor, the Widow and the Wardrobe)

Many creatures had the ability to drain another's life force like Fendahl, (PROSE: The Left-Handed Hummingbird) or the mutated Lazarus, which could leave the victim looking like a dried husk. (TV: The Lazarus Experiment)

The First Doctor's life-force was drained by Mondas during the Cybermen's attack on Earth in 1986. This drain on his vitality triggered the Doctor's regeneration into his second incarnation. (TV: The Tenth Planet)

Abaddon fed off the life-force of living creatures, completely draining it from them. Jack Harkness, being a fixed point in time, had an over abundance of life-force. When Abaddon attempted to steal Jack's, there was too much, and Abaddon was destroyed. (TV: End of Days)

After his resurrection, the Master manipulated his life force after being revived. He was able to jump mind-boggling distances and manipulate electric bolts to fire at people. He did this at the cost of slowly dying and a ravenous hunger. (TV: The End of Time) The Time Lords were later able to fix this problem for him and restore the Master to normal. (TV: The Doctor Falls)

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