Harvester of death and modifier quesstion.

If a modifier on a skill says converts 100% of x damage to y damage.
Does it mean it converts the damage of that skill or does it apply to all such damage?
example : harvester of death

Depends on how the skill is applied. If the skill is a buff or aura (is applied to you), like blood of dreeg while wearing a putrid necklace, then it applies to all damage you deal. If it is a transmuter on an enemy targeted skill like harvester of death or discord for cadence, then it only applies to the skill in question.