So you want to get into Multiplayer Sacred but are a bit confused as to the difference between Open? And what about this "SC" and "HC" you see people talking about? It's all explained here.
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Singleplayer vs Multiplayer
Just why should you play Multiplayer instead of Singleplayer? Well there are a few differences that you should take note of.
Singleplayer has different requirements for moving up in difficulty to multiplayer. When you first start a game with a character, you are given the choice of either Bronze or Silver. In order to get the next level unlocked, Gold, you must complete the storyline in Silver difficulty. This means export your character when you're prompted to in Shaddar-Nur and then you can start a new game with the exported character with the option of Gold. This is required for all the difficulties, so to get to Platinum you must complete Gold, and for Niobium you must complete Platinum. In Multiplayer, you have level requirements instead. Bronze is 1, Silver is 20, Gold is 60, Platinum is 100 and Niobium is 160.
Items and experience are also different. In Multiplayer, if you team up with someone in a party, you will receive only a percentage of the experience for each monster BUT each monster will be worth more experience in total. You also only share this experience if you are fighting within a few 'screens' of each other, so you can team up then hunt on the other side of the world to get full experience bonuses. However even fighting in a team of 4 right next to each other will prove beneficial as you can get through the enemies faster. Teaming up like this also increases the likelihood of rarer drops.
In Multiplayer there is no campaign. I believe you can play the campaign on LAN mode, but not Open or Closed. Also at the moment, the Underworld campaign is in MP but that could still change.
Fun factor. This depends on what is fun for you. Someone who likes to just play on their own, and doesn't even want to join a party will probably not enjoy MP very much. The Sacred community is the friendliest I've seen however, so anyone who does enjoy talking to others while playing will love the multiplayer side. Powerlevellers and loot hunters will also benefit from partying but hunting solo in multiplayer.
LAN
LAN refers to a Local Area Network... in other words this is usually a small group of computers all connected to each other. So if you have two computers in your home that are connected to each other, you can play LAN. By exporting your singleplayer character, or creating a new one, you can play LAN. The gameplay is pretty much the same as singleplayer. Upon death, you respawn at "Haven Isle", the place where you started.
Open
Open refers to a multiplayer game that is hosted on another gamers computer using the Dedicated Server program that comes with Sacred. You use your singleplayer exported characters on this type of multiplayer and Ascaron has no way of policing the game. Therefore you will sometimes come across hacks and dupes. However it can be great fun to try out new builds before using up one of your Closed MP slots. Open does not have the campaign and upon death you will respawn at Haven Isle.
Closed Softcore (SC)
Closed refers to the fact that all the servers are hosted by Ascaron and all the characters are also stored on Ascarons servers. Its much more secure to play (if someone wanted to hack their character, they'd have to find a way to hack into Ascarons system too! possible but very very unlikely) and so is much fairer. Softcore means that you will respawn at Haven Isle upon death. There is a ladder on the Official Sacred Page that records the levels and skills of SC characters. Underworld campaign is available, but the original campaign is not.
Closed Hardcore (HC)
Again, closed means the characters and games are all on Ascarons servers. And like SC, there is a ladder on the life only. So if you die, you have to start a new character from scratch. Sounds nasty, but its actually huge fun. You get a huge rush from taking on tough monsters in hardcore simply because your life truly is in danger, rather than just "oh i'll have to walk back there again" and also the hardcore community is hugely generous and helpful. You can never have enough Life Leech in hardcore!