A D&D 5e warlock takes the Fiendish Vigor invocation.
Lets say the warlock gains 8 temp hp from Fiendish Vigor. Within the next hour takes 8 damage. Then the warlock uses Fiendish Vigor for 5 temp hp.
What happens now:
A. The warlock gains 5 temp hp and can repeatedly gain temp hp during an encounter whenever damage has removed the current temp hp.
B. The 5 temp hp replaces the previous 8 temp hp and the warlock loses 3 actual hp.
You can cast False Life on yourself at will as a 1st-level spell, without expending a spell slot or material components.
Duration: 1 hour.
Bolstering yourself with a necromantic facsimile of life, you gain 1d4 + 4 temporary hit points for the duration.