First off, Welcome to HTS!
Second, hacking a MMORPG is in fact illegal because it is a multiplayer game and hacking it gives you an advantage over everyone else and possibly gives you stuff you would only get through paid membership.
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Now as for hacking games, I don't know too much about that. I know that a classical tactic of disguising amounts of stuff in game is to multiply them by something (e.g. monkey balloon tower defense 4 multiplies your money by 5 in the game's memory, if I remember correctly), And there is also the strategy of having multiple copies of the same value, so that if one changed, the game can sense that, and it changes it back to normal (e.g. gang garrison 2 kept 4 copies of your health in memory, even serverside, which seriously confused me the first time). And, probably the most important point of this, is that you said it was a multiplayer games. Multiplayer games generally don't keep values like that client side, they keep them serverside. You would need to change the values on the server in order to make any difference, or find a way to cheat the system, which is pretty rare. The client side of the game simply shows whats going on in the server side of things, it doesn't actually do anything besides draw cute pictures.
If you really want to get into video game hacking, MMORPGs aren't the first step for you. Start off with very simple client-side games, and then work your way up.
- WallShadow <3