Recently Enma was given to Zoro.
Enma is shown to be an extremely difficult-to-wield sword, with Oden being the only one who ever tamed it. This is because of Enma’s distinct quality to draw out its wielder’s Haki of its own accord, in excessive amounts when swung to force far stronger strikes than intended. It is stated that a normal swordsman would be drained to a dry husk immediately when trying to use the sword, and when Zoro first does so, he cuts a large cliff section off its base despite trying to cut a tree. At the same time, the blade forced hardening on his whole arm and emaciated it.
Usually Enma would have sucked out all of the haki of the wielder, but Zoro was able to stop it and I think Zoro was able to stop it because he has Conqueror’s Haki. He used it right when Enma started to absorb his haki.
What if all Conqueror’s Haki users are able to absorb the haki of others once they awaken it? The absorbed haki makes the user permanently stronger and once they get stronger it’s easier for them to absorb the haki of stronger pirates.
*Theory by Aleeex