Overview
The Most Truest of Basilisks is an unmarked secondary quest in The Witcher 3: Wild Hunt, found just outside Oxenfurt, northeast of Novigrad Gate and southwest of the Herbalist’s Hut. The marker only appears between 7 am and 6 pm, but once it shows up, you can finish the quest at any hour.
What happens
A beast tamer is showing off a caged “basilisk” to a group of Redanian soldiers. It is actually a young, underfed wyvern. Geralt can ask about the creature or point out the fraud straight away.
Once the soldiers realize they are being cheated, they move to grab the showman, and the cage gives way. The wyvern gets loose, and Geralt has to kill it. For a real one, see the basilisk.