Celtic Royal Knight Sword is crafted with Blacksmithing in Mabinogi (weapon). This page lists everything needed to make it, checked against the game as it exists today. The interactive planner lives in the Blacksmith Companion.
How to get the manual
The manual is rank 3 and survives 30 uses; every crafting attempt spends one use.
Gilmore: general shop owner in Bangor, sells them at his counter (117,200 gold).
Materials
Expect about 8 attempts per finished item (13% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
2× Silver Ingot — refine from silver ore. Mine it in Barri Dungeon or north of Tory Ravine
8× Mythril Ingot — refine from mythril ore. Barri Mythril Mine/Advanced, or L-Rod ore deposits
3× Lightless Metal Fragment — Elite Shadow Missions / Advanced Hardmode Dungeons drop
1× Blackened Blade Fragment — Elite Shadow Mission or Alby Advanced Hardmode drop
Finishing (once per item)
2× Tough String — weaving Thick Thread Balls
4× Broken Seal Chain — Elite Shadow Mission or Advanced Hardmode Dungeon drop
4× Large Nail — hit props (lamp posts, signs, statues) or common low-level monster drops
6× Broken Emblem — Hard/Elite Shadow Mission or Advanced Hardmode Dungeon drop