Desperate power

See also: rage (can overlap with this)

For example:
* Gain attack speed for each % of health missing
* Gain attack damage for each % of health missing
* Gain flat number or % increase of either when below certain threshold (e.g. under 25% health)
* Gain access to spell, ability, skill, or such only when below certain threshold (such as above stated below 25% health)
* Game slows down when nearing death
... and so forth.

Does not include items or similar that trigger at low health if they do not do so consistently (as in, they do not break once used).