[Lawtan]

Oddly specific there...hmm...

A servant to an evil wizard lord, supposedly illiterate and weak. However, at nights when the lord is drained, the servant prepares a sleep draught. Once the lord slumbers, said servant sneaks into the library and pours over the forbidden tomes, careful to return them to their original place.
When an adventuring party comes along to slay the evil wizard, the servant tests them. They complete the test, and the servant helps them, seeing in them an opportunity of having the library and tower for himself.