Spoilers regarding the mc's new class (taken from r/sololeveling) :
After clearing the dungeon, the system offers him a class change, to Necromancer. At first he's horrified, because he's a fighter, and believes he should be either a warrior or an assassin. In fact, he hasn't put a single extra status point into Intelligence. So he turns down the upgrade, only to be warned by the system that he's about to turn down a hidden class. Realising how foolish that would be, he accepts the upgrade, only to find out he's earned enough experience to qualify for a double upgrade. The advanced Necromancer class is called the Shadow Lord (or Shadow Monarch) and instead of raising the dead to fight for him, he rips the shades from them as servants. He can raise 30 1st level monsters, or he can sacrifice 20 of them for the possibility of raising a boss to serve him. Deciding to go for the red knight, he takes a chance, and finds out he only has three attempts to enslave a monster's shade. On the third attempt he succeeds, and the blood-red commander, Igris, joins him.