A Threnody is a type of song in Final Fantasy XI. Threnodies all basically have the same effect, but there are 8 of them. Each one of them corresponds to a particular element. When a Bard sings one onto an enemy, the enemy will have decreased resistance to spells and effects that are aligned with that type of threnody.
The threondies are named Fire Threnody, Ice Threnody, Wind Threnody, Earth Threnody, Lightning Threnody, Water Threnody, Dark Threnody, and Light Threnody. The effect of threnody, its duration, and its irresistability can be increased if a bard uses either a Piccolo, Rose Harp, or Mythic Harp to sing threnody.