Elden Ring PC | Crucible Hornshield location & drop from Crucible Knight in Siofra Aqueduct

The only place to get this item is from the Crucible Knight patrolling the bridge in the Siofra Aqueduct, not far from the Aqueduct-Facing Cliffs site of grace. These knights can be encountered as a boss or just a non-respawnable NPC within the world, such as here. This is the slow & safe way to defeat him.

This Crucible Knight drops 3,080 runes on NG as well as the Crucible Hornshield

Crucible Hornshield
"Crucible Hornshield is a Greatshield in Elden Ring. The Crucible Hornshield is a Shield that scales primarily with Strength. Greatshields weigh a decent amount and provide the best protection in the game. Greatshields provide a greater value of blocking and protecting oneself from enemy and boss attacks, but greatly increases the weight and reduces the player's mobility. This type of shield works best at reducing damage or guarding instead of parrying."
https://eldenring.wiki.fextralife.com/Crucible+Hornshield

Crucible Knight
"Crucible Knight is a Field Boss in Elden Ring. These heavily-armored warriors take patience and practice to defeat and utilize power from the Crucible. This is an optional boss as players don't need to defeat it to advance in Elden Ring."
https://eldenring.wiki.fextralife.com/Crucible+Knight