Grave diggers are villagers that can check the role of a dead player every night.
Every night, use the command !check (username) to check the dead of the chosen dead player. The moderator will tell you what the faction is in the morning.
Grave diggers are members of the village faction.
Grave diggers are seen as members of the village by the seer.
Grave diggers are very useful when a player claims to have evidence of another player being a werewolf. If the accused was lynched, you should verify they were a wolf. If they weren't, somebody was lying...
If there is a healer in the game, you can help them pick the right target to heal.