Before you read my solution, I want to say that it's my first time solving any riddle. I don't know how rambling my thoughts are. Even thought there are many clues I couldn't relate to anything (like the third one), I wanted to give it a try.
It's also my first time here, so I don't know what's the etiquette for answering. I do not intend to disrespect the ones that commented and didn't posted any answers by rushing with a silly answer. I am sorry if I had to be more sure before answering.
Right as he gave his last breath, he said, search, seek and destroy.
I Google searched "Seek and Destroy" which is a Metallica's song. (Daniel C. comment)
The gold is
Master of Puppets, third Metallica's album.
The acrostic you made (pointed out in Ewasted's comment) reads "Three". Master of Puppets was Metallica's third album.
It was the band's first gold album.
The fallen leader spread a plague that turned people into the undead.
This could be a reference to the album's cover: tombstones being pulled by strings (the fallen leader being the puppet master).
He got tricked by Xavier Fisher, who took all his gold.
You said "Xavier Fisher" encripted tow letters: Xavier and Fisher both end by "ER", which could stand for Elektra Records, the record label that owns (stole?) this album.
as you suggsted in you first clue, Xavier is where the gold is.
What's next feels extremelly forced to say so it must be wrong, but I couldn't think of anything better. The gold is on the National Recording Registry (it seemed a distinctive place for a music album to be). Xavier must be there?