All of the Demigods have something they are known for. Radahn is known for his indomitable strength, and is called the strongest of the Demigods. Malenia is known for her unrivaled speed and unmatched dexterity with a blade. She is known as the Demigod who has never known defeat. Ranni is known as the smartest of the Demigod children, and cannot be bested in a game of wits.
Related: Elden Ring: What Is Volcano Manor
On the other hand, Rykard, before he became the Rykard we meet in the game, really isn't known for anything. He doesn't possess the strength of Malenia or Radahn. He doesn't have a penchant for blood like Mohg. He can't even be considered overly faithful, like Miquella, since Rykard's whole character arc centers around heresy. It should be no surprise then, that this scorned son of Radagon decided to make his own power and take it for himself.
The being we meet deep in the bowels of the Volcano Manor is actually not Rykard. Or more specifically, it is no longer fully Rykard. The abomination we face off with is just that: An unholy fusion of a Demigod and a world-eating serpent. Before Rykard became what we know him as, he was a fair humanoid, though despite his outer beauty, he always housed an ugliness within him.
Rykard, Lord of Blasphemy, was originally born Praetor Rykard to Radagon and his first wife, Rennala. It does not seem that Rykard was born with any special abilities like his siblings. As such he took off into the Lands Between and discovered a civilization on Mt. Gelmir. Here he took the title of kings, and he met a dancer by the name of Tanith who he took as his consort. It seems that the only godly ability that was imparted onto Rykard was an infinite lust for power, and an unquenching jealousy
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