The message "[monster] appears out of strange mist" isn't quite correct. It should be changed to either:

1) "[Monster] appears out of a strange mist" ("a" added before "strange mist).
2) The description of the summoning spell mentions creating a strange mist, and the message is "[Monster] appears out of the strange mist" ("the" added before "strange mist"). However, to do that, you'd need to change the description of the spell if the summoner was invisible or out of sight to mention a strange mist appearing from out of nowhere.