Hello Juju2112,
I have ran the adventure as a DM and I have not found the answer on your question within the PDF-file. I think it is up to the dungeon master to design this. Ofcourse, you can always answer this question with like: It's magic. It works in mysterious ways. I do not think it matters that a 4th level necromancer played by the dungeon master need to consider the rules step by step. I mean, player characters have levels, and most of the time a monster has got a CR. What I think is important is that Beryn needs a story, from where the player characters can conclude: This must be a powerful necromancer, for he can summon the dead! And do not think in player game terms: Oh, he must be over 5th level at least to cast the raise dead spell... As a dungeon master, you could come up with anything. Maybe Beryn had found a tome, telling him secrets to command the dead. In my version, Beryn was like Frankenstein, experimenting on beings, trying to create life, trying to solve the mystery of life. Accidentally, he created undead. I think it's important that it's correct, storywise, and leave the game mechanics for player character creations only. If you want to read more about my version of Beryn, see 'My Farland Pen & Paper Game' topic, first adventure. Good luck on your adventure!