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My mage became Level Drained after fighting a few undead creatures.
What caused this to happen and how can I bring her level back up?
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you need to use either a Lesser or Greater Restoration spell. clerics of a certain level have these spells (the Lesser sooner than the Greater, obviously) but you can go to any temple in the city and pay for a Restoration. you can also buy Restoration scrolls at various temples...a good thing to carry around for random level-drain instances such as the one that happened.

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Nevermind, I used a Greater Restoration spell and she seems fine now.
It must have been caused by Vampires or something.
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You can use the scroll (buy from shops), use the memorized spell if your chars can cast it; and also just go to temple and get the service, which though can be at first quite a whack at the wallet (700 gold). But there are ways and yes, vamps can drain the level, and few other 'vampiric' things.