Resonant Weapon is a Hex in Dark Souls 2. To cast a Hex, you must use a staff/chime or Special Weapons that can cast Hex.
"A hex developed independently by Outcast Felkin. Transforms a certain amount of souls into attack power over a period of time.
Something gained, something lost.
Catalyst: sacred chime"
- Felkin the Outcast for 4000 souls
- Costs 2000 souls for maximum damage. Buffs weapon by adding 50 dark damage, then multiplying total dark damage by 1.35X. Lasts roughly 60 seconds. Does around same damage as Dark Weapon at 2 Attunement slots cost, shorter duration and soul cost.