Tier 0 - Solitary Magic
When you start the game, your first major achievement will be reaching Tier 0, which begins after you end your apprenticeship. Most players will reach this tier by ending their apprenticeship or slaying their Master to gain the Adept class, but two other secret classes exist. The Guide goes over this phase of the game in great detail, but it comes down to four ways to reach Tier 0.
Your choice of class at this point has little impact on the rest of your run, but the secret classes decrease your maximum skill level with several skills while increasing your maximum with others, so think carefully before selecting them.
These classes are mutually exclusive. Selecting one class will lock the others, grant the tier bonuses below, and advance you to Tier 1.