I have noticed that the new Oathkeepers damage buff is not applying to Leviathan's Breath, at least on Kalli. I have tried Wishender and it worked just fine. I also don't know if this is just a Kalli thing or not, so a bit more testing needs to be done.
If Bungie said anything about the buff intentionally not working on Leviathan that's my bad and pleas let me knwo but I have not found anything in the last TWAB so yeah.
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The issue is you man. I used leviathans and oath for a solid 2 hours, it was working completely fine. Damage would go from base 45kish to around 69-75k at max.