About
Guild Charter is a custom item made especially for Blackblock.
Season 7 note: the Guild Charter is not yet available in Gypsum.
Explanation
Guild Charters allow you to create a guild!
At least three players are needed to start a guild. (If you really, really, really want to create a guild with just two people, open a support ticket and a mod can help.)
When creating a guild, you can register a banner as your guild flag. Guild members have their guild flag displayed next to their name in chat. Note that the banner will show up horizontally, so keep that in mind when designing your flag!
Guild members can share a Guild Chest, which is like an ender chest that is shared between the guild members.
You can join up to three guilds, but you must pick one to be your main guild. (This determines the guild flag that shows up in chat.)