Best action RPG I've played probably ever.
I'll give you quick answers, others will say more
1. Online is very different but you can basically play it single player if you don't want help. Other people can only spawn to your game when your are alive which is not often ;)
2. Yes it is a hard game, it doesn't hold your hand. You will die and you will be forced to learn the combat system to proceed. The game really forces you to master it if you want to beat it.
3. Most of the time you are a soul trying to get your body back. When you beat an end boss, you get your body back and can then have other people join ur game to help you, if you want. Then when you die you have to fight for your body again. Hard to explain but easy once you see it.
Just get the game, its freaking awesome if you like action rpgs :)
Current Favorites:
Wii - Muramasa, MH3, Metroid Trilogy, RS2, Okami, RE4, COD, Dead Space, Scarface
PC - L4D, TF2, BFBC2, Killing Floor, Aquaria, Warsow, many UDK indie games
Mods - MechWarrior:Living Legends, Star Wars: Galatic Warfare, PVKII, Goldeneye Source, L4D mods
PS3 - Demon Souls, Way of the Samurai 3
Social Modding Club: ModDb | Steam
