One of Halaster's apprentices was a human transmuter named Arcturia, who created all manner of new monsters and often made herself the subject of her own horrid magical transformations. She claimed an entire level of Undermountain as her lair and polymorphed prisoners into monsters to guard it. It's likely that Arcturia resides in Undermountain still, in one form or another.
Arcturiadoom
CardWDMMp. 3192014
Raw data
{
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"entries": [
"One of Halaster's apprentices was a human transmuter named Arcturia, who created all manner of new monsters and often made herself the subject of her own horrid magical transformations. She claimed an entire level of Undermountain as her lair and polymorphed prisoners into monsters to guard it. It's likely that Arcturia resides in Undermountain still, in one form or another."
]
}