You as a Nix can tame exotic animals and monsters like Ash Crawlers. All Nix have this ability regardless of what type they are. This is done with a Animal Handling check you are proficient in. If you succeed it becomes a friendly mount permanently until it dies and will fight on your initiative. If a Ash Queen is present she can override your check. You can however very rarely take a Ash Queen. If this is the case her entire pack of Ash Crawlers becomes friendly towards you. If the Queen dies the Ash Crawlers fight to kill what killed her then go into a blind rage and fight everything else around themselves this is because they've gone into fight or flight realizing they don't have a Queen and can't reproduce.
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