The Serum of Immortality was created by Castillo Sermano.
Castillo created it to live forever.
However, the side effects turned him into a zombie. He uses it to create zombie to control.
Over time, Castillo realized that the serum had flaws and it won't protect him from time. Over time, his body became to deteriorate and his body is making slowly and weak. Castillo realized that he'll die despite the serum. Castillo and his zombies murdered anyone who came to the island. He used the dead appendages of the the victims' bodies to put on himself so he can continue moving faster while his zombies became slower, weaker, and eventually die (for good) while he lives.
However, Castillo eventually died at the hands of Alicia.
However, Rudy takes Castillo's research which causes further gruesome events in the future.
Dr. Curien carries on his work by taking his research and the serum and this leads to the events of the second House of the Dead film which is based on the House of the Dead game series.