Scenario review please! The Necromancer's Tome (insert dramatic music)

[Corvus5200]

This was the center piece for our 4 player game last night. It is a lead in for Thaw of the Lich Lord and how he is awoken. That being said, we will have to play it again because we ran out of time and no one reached the objective... Please check out the description below and give it a critique. Let me know if you think it is balanced. Thanks for your input!
Scenario:
There is a central tomb/altar/ice cream shop that holds a special grimoire of Control Undead. It was written by such a powerful mage (the Lich Lord) that it already comes with a +2 advancement (+3 if you already know it). The mage or his apprentice needs only to spend one action on the top most step and then will receive the book. (50 xp, as well) Now, no necromancer worth his mascara and Cure albums would just leave a book like this laying about! So, instead, every round, at the beginning of the creature phase, an undead (or two) spawns out of this cradle of evil. (We used cards to keep it simple. I used almost every undead in all the expansions and used a card with one and a card with two of the weakest. It also increased the inherent possibility of the low tier guys spawning.) They will act as normal monsters. Each player will also place two treasures as normal, with wandering monsters as per normal.