"In this world nothing can be said to be certain, except death and taxes."
So said Benjamin Franklin in a letter in 1789.
In Game of Thrones, there is an equivalent.
Valar morghulis is a High Valyrian saying originating from Braavos, a city located on the northwestern tip of Essos. It means "all men must die," and it's usually answered with the phrase valar dohaeris, meaning "all men must serve."
If Morghulis looks appropriately ghoulish, it's probably because it resembles the English word morgue - though we don't know if George R.R. Martin intended that or not.