Free addictive muliplayer RTS RPG cross
The difficulty setting here is of no sense because it depends on how good you are against your human opponents.
Graphics are a bit outdated but they are adequate
Sound again is adequate
The game mechanics are fairly simple and there only few arena's but it shines through its simplicity because the only thing that matters is your and your team's skill.
I will not bore you with details of the game since you can try it out for free but I can tell you it is absolutely worth playing it especially if you like PvP games.
Also there is a very wide selection of champions to suit your playing style. For example a player may like a nimble harasser/troller that deals heavy damage like me better than a caster or a supporting class. In essence there are champions to suit every player style as it is important to feel comfortable with your champion and know his strengths and weaknesses.
Just try it out and you won't be disappointed.
P.S.: if it was a paying game I would consider it a lot but since it is free TOTALLY WORTH IT