Omeluum needs Underdark components. The Underdark has Timmask and Tongue of Madness plants. They're rare and scarce due to their dark growth.
Timmask plants, like Tongue of Madness, are found throughout the Underdark, but the Arcane Tower holds numerous pieces of Timmask Spore. Clearly, a potions-making spellcaster owned the Arcane Tower.
When you return with all the components to Omeluum, he will create a delicious beverage for you to consume.
The concoction will weaken the parasite's defenses, allowing Omeluum to penetrate your brain psychically and take control of the worm.