The ghosts in the mist don’t matter – ignore them. If you draw a tile with mist on it, you now have the option of adding ghosts to the game. How exactly depends on how you place the tile:
• Expand the mist: You place a tile with mist on it such that at least 1 side expands an existing mist bank. You must then add 1 ghost to another player’s meeple on the game board. It doesn’t matter if you finish the mist bank. You just have to expand it.
• Limit the mist: You place a tile with mist on it such that at least 1 misty side is placed next to an existing, mist-free, tile. In this case, you must then add 1 ghost to one of your own meeples on the game board.
• It can happen that you simultaneously expand one mist bank while limiting another. In that case you must first add 1 ghost to another player’s meeple on the game board, after which you must add 1 ghost to one of your own meeples on the game board.
If the general supply of ghosts has run out, you don’t need to add any ghosts.
Or, the other way around: If there aren’t any meeples you can add a ghost to, then you skip step 2a.
As soon as you add the third ghost to a meeple on the game board, you must return the meeple and the ghosts to their respective supplies.
It doesn’t matter whose meeple it is.
You limited the mist and must therefore add a ghost to your meeple . Since this is the third ghost, your meeple is terrified and runs away. In other words, you return it to your supply, while also returning the ghosts that were haunting him to the general supply.
After potentially placing ghosts, you may place one of your meeples onto the just-placed tile in accordance with the base game rules.