Compared to the other two factions, “Adventurers” sounds like such a generic name. Who would want to be an Adventurer when they can be an Immortal? Well, everyone has to start somewhere.
The Adventurers faction isn’t so much a faction as it is the default state of all Diablo Immortal players. As soon as you step off the boat onto Wortham after creating your character, bam, you’re an Adventurer. As Adventurers, players have access to weekly bounties that let them slay high-priority targets for fancy rewards, but that’s about it. Unlike the other two factions, Adventurers don’t get any big boons, but they don’t have to worry about upkeep, either.
While Adventurers can eventually graduate to Shadows or Immortals, they don’t have to. Anyone who would rather log in every now and then to complete a few missions will probably be satisfied with the Adventurers faction.
Diablo Immortal players who are in the mood for a bigger challenge, as well as an ongoing PvP war, can join either the Shadows or Immortals factions. Of the two of them, it’s easier to become a Shadow.
Before players can actually join the Shadows, they need to be at least level 43 and have finished the Bilefen questline. Once they do, they can enter the Shadows Lottery. Every day at 12 pm, 6 pm, and 9 pm server time, Shadow hopefuls can talk to the Mysterious Patron at the Westmarch Tavern. If luck is on their side, he will invite them to a room behind him to fight three Ancient Champions. Players who come out victorious will be full-fledged Shadows. If a Shadow aspirant isn’t invited to fight the Ancient Champions or cannot beat them, there’s an alternative solution.
While most players will probably join the Shadows faction through the lottery, existing members can invite friends, family, and even random strangers to become new Shadows. This requires an Akeba Signet, which is consumed upon inviting another player to prevent Shadows from spam-inviting an entire server’s worth of members. The only way to get a new signet is by going through the Shadows Lottery as a member and defeating the three Ancient Champions again.
Source : https://www.denofgeek.com/games/diablo-immortal-factions-immortals-shadows-adventurers/