Poison dot blessing - adeptus biologis neurotoxin, some poison from catachan jungle etc. name it as you wish. Or maybe it should be a passive ability of this weapon, like Kerillian’s daggers got. Or maybe some debuff like slowing enemy move/atack speed/chance to paralyze human sized enemy (similar to surge stuff effect).
There are poison blades in the Catachan arsenal, but they are more daggers than Swords