|Stone of Righteousness|
The Stone of Righteousness, more commonly referred to as the Truth Stone, is an ancient stone that grants it's holder immortality and protects them from fatal injuries. Only the Nether Saber is powerful enough to permanantly kill whoever holds the stone.
After Nox Decious had his face burned and realized he was mortal, he created The Darkness to bind the Stone of Righteousness to his soul and in doing so, Decious became immortal. During his return he used the power of the stone to Resurrect Ganondorf.
The Stone of Righteousness is revealed in "This time, It's Personal!" by Wario. When he stabbed Nox Decious with the Nether Saber in "The Most Epic Battle EVER," it separated his soul from the Stone of Righteousness. Wario then picks it up and decides to keep it for himself.
As well, the Stone of Righteousness helps with Link's resurrection since Merlin used the last bit of power that the stone had to bring him back to life with.
Wario had eventually obtained the stone afterwards and told Luigi's son stories of a man made of shadow, the Darkness. At some point Luigi's son retrieved the Stone Of Righteousness and gave it to Mario, saying it was the reason why Wario could defeat The Darkness. Mario realizes the answers to killing the Darkness are in the stone, and remembers the old saying "In the Darkness Lies the Truth" and thanks Luigi's son for giving him new confidence.