This page of our guide to Greedfall describes the walkthrough for Footsteps of Madness, one of the main quests.
After relieving Hikmet and San Matheus, return to Constantin and talk to him about the situation. The man will start to behave strangely and will suddenly disappear. Ask the guard where Constantin disappears, and then check the records in his office. Now go to Cwenvar and ask the local natives if they saw Constantin.
Hide in a safe place and wait for the cousin of the main character. Then, go down and follow the man - defeat the beast, and watch a cutscene. As it turns out, Constantin is the culprit behind the attacks on the island!
You will have to battle a wraped variety of Nadaiga mageman. The creature uses a spear in combat, which gives it great range, and through corrupted power, it's even stronger than ever. You have to remain cautious - a few hits and your character can say goodbye to life. Patiently grind down the enemy and focus on evasion, and after a while, you will win.
After the battle is over, watch a short cutscene and your mission will be completed. You get 2000 XP and begin the Break Ties and Together for Teer Fradee quests. In addition, your relationship with the Congregation of Merchants will become hostile.