The Hex Master is a Coven role with the ability to Hex one player each night.
When all the non-Coven players left alive have been hexed, all Hexed players will die (in an Unstoppable attack).
Players are not told that they are hexed. Hexing a player can be stopped by a Bodyguard. You will trigger a Trap but won't die from it.
Players who are hexed will appear to the Sheriff as a Coven member.
Players who are hexed (or framed, for that matter) will show up under the Hex Master's investigative results.
With the Necronomicon, you are no longer considered visiting anytime you Hex so you can get around any role that sees visits or attacks visitors.
This means that the Hex Master with the Necronomicon won’t die to a Werewolf that attacks their target, and can get around the protection of roles like the Bodyguard. And it will deal a Basic Attack to them - people that were previously Hexed will still be Hexed.
Don't go down or up the list starting from top or bottom, as these players often get targeted by other killing roles, or worse, protected by a TownProtective role or is an alerted Veteran. Picking random people with a name that doesn't stand out means you'll be able to win quicker.
Keep track of the people you have hexed and share it to the Coven members. Kill the ones who are not hexed to end the game quickly.
As the Coven is also capable of killing, ask your Coven to not attack the person you've hexed in order to not perform any useless work.
If you know that the Coven is in danger of losing, use the Hex Master as a bluff, say something along the lines of, "Everyone is hexed except [insert player's name here]!" Using this threat, you'll be able to trick the Town into doing your bidding in exchange for not Hexing the "last not Hexed player". This creates panic, and diverts attention from the Coven.
When choosing what people to hex, follow the strategies of an Arsonist, since both roles work in the same way.
Choose people who you think will live the longest. Remember, when every living Townie becomes hexed, an Unstoppable Attack happens, purging the rest of the Town in late game. Even if there are only 3 people that oppose you, if you guessed that they were going to live until the end of the game and hexed them earlier, you could still finish them off, which would ensure your victory.