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Why didn't the Magus just throw the Grimorum into the body of water the group was in before entering Avalon or burn it with a torch before entering Avalon, instead of forcing Finella to go into a life of hiding from Constantine and forcing Mary to do the same AND abandoning her son?
And, how could Mary just abandon her son like that? I mean, I've heard of the "Parental Abandonment" trope being used for the sake of good drama (and it did turn out to be good drama in Gargoyles), but I find it a little ridiculous that she believed that leaving her only son at such a young age was the best course of action.
Oh, and here's that trope if you're interested (I <3 TVTropes):
Destroying the Grimorum isn't that easy. And given that fact, the need to protect it and thus protect Tom from being attacked on Avalon by Constantine made Mary's decision necessary. It wasn't abandonment at all. It was her staking out a defensive position to protect her son.