Ok first off to enable monster play you have to get your charcter upto level 10 (any character can enter monster play), once you have acheieved level 10 you have to go looking for a Fell Scrying pool, these are available throughtout middle earth Bree & Thorins gate) and are shown on your in game map.
here is what a fell scrying pool looks like.
to activate this all you have to do is right click on it like you would with any other action in LOTR, Once you have activated it you will be presented with a choice of monsters.
once you have chosen your monster you will be taken to a completely different part of middle earth where you will now control your lvl 50 monster!
Now the fun bit about this is the fact that you have quests and suppliers (much like you would have them as a good guy).
So some of you may be asking what is the point in monster play, Well monster play allows you to build up your destiny points when you are combat instead of earning XP you earn destiny points, these can be used on your main character or they can be used to build up your monster play.