Arthur was granted access to the shards after his resurrection, when Excalibur was corrupted by the Arcane Order's dark magic.
He later critically wounded Jim with a shard when they were fighting below Hoboken. Merlin was forced to place his fallen champion into stasis to prevent the shard from crawling its way into the boy's heart.
When Arthur and Morgana fought in what was to be their last battle, Arthur lethally wounded Morgana with a shard, but not before she wrested his control over Excalibur, thus stripping the sword permanently of its ability to produce onyx shards. Jim was even cured from his corruption and reverted back to his original human form, thanks to Claire's shadow magic.
As the name implies, onyx shards are irregularly-shaped black crystals.
The shards are able to corrupt a victim's heart if it strikes their in the chest, even a considerably strong-willed individual, like Jim. Once it completely enters the heart, it turns the victim into a monster, effectively turning the victim into a brainwashed slave under the user's control. According to Merlin, there is no known cure to remove the shards (or more specifically, an easy way).
The shard that affected Jim even prevented him from summoning his weapons, leaving him virtually defenseless.
The only known ways to pause the onyx shard's process is by placing the victim in stasis within a crystal or, in Jim's case, traveling back in time to freeze the corruption temporally (though they will still remain critically injured).
The victim can be freed completely from the corruption caused by the shard only if its user is killed (in this case, Arthur).
|Appearances in the Trilogy|