Vlark is an urbane monster in the Canal Town of Kaiin, who struck some kind of bargain where the Prince's men don't hunt him to death, and in exchange he doesn't eat more than 6 people a year. Vlark is droll and clever, but also overwhelmingly powerful, so he leverages this bargain to his advantage by scaring the bejeezus out of the people in Canal Town.
In 5e Dungeons & Dragons
As originally presented in The Kaiin Players Guide, the Vlark is a unique but terrifying monster. I'm going to make Vlark the name of a rogue mind flayer who happens to hang out in Canal Town. It's not entirely clear why, except that maybe it likes having humans live in fear of it. If you're highly resistant to magic and an overwhelming bad-ass, hanging around an extremely decadent, urbane society with an advanced economy isn't a bad deal, overall.