The Riven you forma with the appropriate polarity for your own riven.

Slash is used for the ability to create body parts ([Nekros] can loot body parts as if they are separate corpses but they still count as 1, leading to more resources)

For statstick, you need:
-any weapon with high dispo (1,45-1,50 ideally)
best rolls - crit chance, crit damage and damage
good negatives - range, attack speed, crit on slide, damage to faction, finisher, heavy attack, impact, puncture