Level 1 boss
LEVEL_1_BOSS49.4%

Achievements and their recorded unlock rates.
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LEVEL_1_BOSSMEMORALIA_FIRSTLORE_BOOK_FIRSTLEVEL_2_BOSSLEVEL_3_BOSSLEVEL_4_BOSSLEVEL_5_BOSSLEVEL_6_BOSSIMMORTALMEMORALIA_ALLLORE_BOOK_ALL