'Enhances "Souleater" effect' is an additional 2% HP converted to damage during Souleater. Souleater converted HP percentage caps at 12%. Without any items, Souleater is @ 10%, so it only takes one 'Enhances "Souleater" effect' item to cap it.
This particular item is the preferred 'Enhances "Souleater" effect' item for DRK zerging with multi-hit weapons, because it occupies a slot which doesn't have any haste items.