Call it what you want. Suggestions: "Dampened", "Fatigued", or "Wounded"
Add this condition to select abilities, especially lack luster ones, to give them a utility use within kits and perhaps have this be a piece of identity to some masteries
It will function just like a DoT; Applied through ability use (maybe even some niche weapons could have a % chance to apply the condition added to their swings), it would be cleansable with status extraction, and last X amount of seconds after applied. Pre-existing instances of it would be overwritten if another instance of it is placed on the same enemy. Same as other conditions.
Wounded I - Healing Received reduced by 5%
Wounded II - Healing Received reduced by 15%
Wounded III - Healing Received reduced by 25%
The particle effect could be some semi-translucent grey bars that "rain" down the bottom half of the affected character. Giving a feeling of "draining" - that will provide a visual queue to what characters are under the condition. (this can really be anything. this is just what comes to mind if I force myself to offer an idea and obviously the art team wouldn't use MS paint like I did. They could take my effect idea and make it actually look good. Just trying to offer some concept for the vision)