The Lothric War Banner's weapon art has a long AoE buff that has some unintended consequences when active. Here's a list of effects I've experienced:
-Illusory walls will be revealed when they're within the radius of the buff
-If you have a weapon that has an on-hit effect (not consecutive hits like Pontiff Eye rings or Old Wolf Curved Sword) in your right hand and the Banner in your left, while the buff is active, the effects will constantly proc. Handmaid's Dagger will regen FP, Butcher Knife will regen HP, Great Corvian Scythe will inflict constant bleed damage, and Chaos Blade will constantly inflict damage.
-The AoE effect increases your detection radius. Even when wearing the Slumbering Dragoncrest Ring, if you approach an enemy while the buff is active, the enemy will notice the perimeter of the AoE and turn around from much further away.
For the record, I love this weapon, but man, it is goofy. I don't know if I've even scratched the surface of how weird the buff can get.
EDIT: Not only does the Handmaid's Dagger and offhand War Banner WA proc the FP regen, it also had a visual effect that was the same as Darkmoon Blade. Thanks to /u/WilkestheChops, I now know that it actually is an additional unknown AR buff that stacks with the Banner WA, although at this time I can't find what it scales with, or if it even scales at all. More importantly, the unknown buff can be transferred from the two-handed Banner to the one-handed Dagger, and it persists for some time after the Banner's WA expires.