The Crucible Knight is an optional Field Boss introduced in Elden Ring. You don't need to defeat this boss to progress further into the game. The Crucible Knights served Godfrey, the first Elden Lord.

Aspects of the Crucible: Tail is an Incantation Magic Spell in Elden Ring ... by a Crucible Knight in the Stormhill Evergaol. Please note the location indicated on the map image below.

Elden Ring follows the same formula ... to learn where Miyazaki and gang got the inspiration for the famed Crucible Knights from. As mentioned by TheGodskin's post on Reddit, the Crucible Knights ...