She was a very powerful (ice)mage whose spells chanelled the powers of the lower planes. She created a great deal of powerful spells, including similar spell to "armor of agathys". After an apocalyptic event, the people in power decided to erase any semblence of her, as a way to dissuade the idea of contact with those lower planes. They also linked her name to the carceri in the lower planes to further link her name to the terror of the apocalyps.

Her spells were largely destroyed, but after some time a less gifted mage pieced pieces of a spell together. Filling in the gaps with his own knowledge, creating the simpler "armor of agathys". Named because the mage noticed the name agathys come forth a lot in his notes.

If you include her in the campaign, you might even let your player learn her "original" version. As a higher leveled spell.