[@Dead Cruiser] Nature magic is the least regulated in terms of a system- its essentially d&d druid magic, with a bit of a darker twist instead of just plants and animals and stuff. You can manipulate nature to make dark roots pop out of the ground and grab people. You can tear and steal people's shadows to harm them/heal yourself. 

As far as "monster transformation" specifically goes- Its pretty much up to you how it actually 'happens'. It could be dark magic circling around you and clouding you from view before making you a monster, the rapid growing of fangs and a snout, anything really. Rule of thumb with nature magic, if you can make it appear suitably eerie/dark/creepy it'll probably be fine.