This ability is deadly when cast by enemies. As the spell description says, it prevents all healing, from regeneration to healing spells to poultices, and even the Lyrium Veins found in the Fade. It also does significant damage, so use extreme caution when facing an enemy which can use this skill.
Thanks to a bug (see below) the curse doesn't actually prevent health regeneration. That means that Regeneration (spell) can actually be used to keep someone hit by the curse alive... but be very, very careful with them until the curse goes away.
This spell does not prevent regeneration in combat due to a script error, it only prevents health regeneration outside of combat, which renders this part of the spell useless, because both party members and enemy spellcasters will always enter combat mode as soon as they see enemies. Unofficial fix for PC is available here. The spell still prevents regular healing and inflicts damage, however.