Demonic Death Penalty Blade 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 1 and survives 30 uses; every crafting attempt spends one use.
The Secret of Scathach Caverns: Lord mission from the Tara board. Drops Demonic weapon manuals from its reward pool.
Materials
Expect about 10 attempts per finished item (10% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.