1. Using "that" as the conjunction:
* The fact that death is certain is undeniable.
* It is a truth universally acknowledged that death is certain.
2. Using "whether" as the conjunction:
* The question of whether death is certain has been pondered for centuries.
3. Using a wh- question word as the conjunction:
* How death is certain is something we all must face.
* Why death is certain is a mystery that has baffled philosophers for ages.
4. Using "if" as the conjunction (in this case, "if" functions as "whether"):
* There is no debate about if death is certain.
The best way to write the sentence depends on the context and what you want to emphasize.