The Oath of the Skull
The Oath of the Skull is the oath that each Phantom swears as he enter the service as tha new Phantom. To date 22 people are known to have sworn it; the Phantoms from the 1st until the 21st and Julie, a.k.a. "the Female Phantom".
The original wording given in the first Phantom story The Singh Brotherhood was "I swear vengeance against the Singh, against all piracy, greed and cruelty. I swear that as long as my descendants walk the earth, the eldest male of the family shall carry out my work."
Subsequently the part about vengeance against the Singh in particular was dropped. The wording of the oath kept varying slightly from story to story and the most common variant is "I swear to devote my life to the destruction of piracy, greed, and cruelty, in all their forms, and my sons and their sons, shall follow me."
The Oath is usually sworn by the son when the father is dead or dying. Team Fantomen has it that the father could be incapacitated. The Oath is sworn with one (or both) hand on the Skull of the killer of Captain Kit Walker, the father of the 1st Phantom.
First appearance of the Oath of the Skull on October 7th, 1936.
Team Fantomen stories
The 17th Phantom swears the Oath.
The Oath as shown in a later story, "Gold of the Green Hell"