Memento mori is originally a latin phrase with a meaning of “Remember Your Mortality”
Plato  introduces the idea that the proper practice of philosophy is "about nothing else but dying and being dead".
The Stoics were particularly prominent in their use of this discipline ,the Stoic Marcus Aurelius invited the reader to "consider how ephemeral and mean all mortal things are"
A version of this is often rendered into English as "Remember, Caesar, thou you are mortal"
The expression memento mori developed with Christianity, embracing Heaven, Hell and salvation of the soul in the afterlife.