@Alzeroth: Assuming the Langdebeve's is the same "Defense Down" as the Stribog, it does not stack, and will not be overwritten by Angon. I don't know the potency of Stribog or this one's effect.
This is currently also the best polearm for a Drg right now.
The highest level of Oathkeeper is dmg 101 Delay 492. And since the next level for Oathkeeper is Rhongomiant (good luck ;-;), you're better off going for this thing. Same stats with the bonus of Weakens Defense +15.
Whether the 15% from this weapon stacks with the 20% from Angon. I am still not sure of.