Dragonspear castle basement Legacy of Bhaal event (spoiler)

After you interrupt Hephernan from some ritual is this an event you're supposed to escape or fight? Because the waves seem to endlessly spawn. Is this a bug or a change for the Legacy of Bhaal setting?
I don't understand because even if it's not a bug how are infinite waves of spawn viable? Especially on LoB
I don't understand because even if it's not a bug how are infinite waves of spawn viable? Especially on LoB
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I don't think the spawns are actually infinite though.
the level 5/level 6 elemental summoning spells are your friends, especially if you have the level 6 druid ones