It was created by a few of the people from Blizzard North (makers of the Diablo series).
If you like Diablo, you'll probably like Mythos. The game is fast-based, which means that u can fight 20+ mobs with no worries, not like in WoW where u gotta run if u fight 2 mobs.
And basicly everything else with the game engine is like Diablo, but the graphics are a bit more peaceful, and all chars are not humans, you may choose from Elves etc.
It is absolutely free, (but!) if I remember correct, it's still Closed Beta (u can only play Mythos if u get an invite). But nowadays its not hard at all to get an invite.
And when they release the game as a "real" game, they plan on making it free to download, free to play