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).
Possible way to buff catachan swords (e.g. kerillian's daggers)
There are poison blades in the Catachan arsenal, but they are more daggers than Swords