You do not need to learn any of the Unforgivable Curses to get the Spell Master (silver trophy) achievement during your playthrough.

Hogwarts Legacy Permanent Events and Missable Features Does not count towards getting the Spell Master (Silver trophy) achievement.

There is no other way to learn these spells once you miss them so you will need to reload an earlier save and go through the dialogue choice again. Note that all Unforgivable Curses (except Confringo) are permanently missable if you respond with the wrong dialogue choice during their respective questlines. Hogwarts Legacy All Companion Relationship Lines List and Guide Permanently missable if you refuse Sebastian’s offer to teach them to you. Afterwards, you must complete side quests specific to his Relationship Line and answer with the correct dialogue choices to obtain Crucio, Imperio, and Avada Kedavra. The Unforgivable Curses can only be learned after befriending Sebastian Sallow, a student from Slytherin House. Hogwarts Legacy All Spells List Befriend Sebastian Sallow and progress through his Relationship quest lines. Confringo, referred to as the Blasting Curse in the Dark Arts Battle Arena, is categorized under Damage Spells in the game. There are a total of 3 Dark Arts spells you can obtain throughout the main story which are Crucio, Imperio, and Avada Kedavra. Hogwarts Legacy Best Spells Three (3) Learnable Unforgivable Curses in the Game Most Dark Arts spells inflict the Curse status on targets that causes them to take extra damage from subsequent attacks while Avada Kedavra, the strongest Unforgivable Curse, can kill targets instantly in one hit. Dark Arts Spells or Unforgivable Curses are a category of forbidden dark magic in Hogwarts Legacy.

Hogwarts legacy dark arts consequences