Well, only certain masks have attained the power to be able to gain control of their hosts. While wearing a lesser mask only gives them evil intentions. Like maybe the mask of ice, (just an example) if the user uses it for an extended period, the mask will be able to fill the user with it's own intentions making them join the Council. Some masks, have grown their powers over hundreds of years to be able to fully control the user by filling them with evil intentions. Some masks search for their future hosts while some wait for the host to find this.
Example: Treasure Hunter Joey walked through the forest. He arrived at a forest clearing. Finds mask on floor. Picks it up. Senses a presence telling him to put it on. Joey puts on mask. Joey feels more powerful. Mask searches Joey for any evil intention. Finds some evil. Few weeks later. He is welcomed into the Council.
Stronger Mask Example: Man looks in front of him. Sees floating mask. Feels irritating pain in head. Gets blurry and loses conscious. Few hours later. Wakes up in bed with another woman. Lmao, jk. But see the difference. The second mask is more powerful but it searches for it's own host.
More backstory: The original 18 users of the mask were pure hearted and were able to gain control over the masks. They knew the evil presence within and the dangers it could pose on the future world. They all agreed to go their separate ways with the masks around the world to hopefully never have the masks come in contact with each other again. When all the masks are together an even greater entity of evil could be created and used to cause death... and rebirth. They were then passed through generations throughout each family. Over hundreds of years, however, some masks were able to gain control of the descendants and started their goal to reunite together once again, using their hosts to find the other masks who waited in peace. And eventually use their power to once again, complete their goals. And during the present day, only a few masks without hosts remain, waiting to be found by the masqueraders as they are now called to bring their power together and complete their goal of whatever it is.