There is a dungeon in Infernax called Stormheim Castle which in order for it to appear you must first solve a puzzle. This How to Make Stormheim Castle appear guide will tell you the solution to the puzzle so that you can access the dungeon.
Midway through Infernax when you have found the dam and broken it you will learn from Harry in the tavern by the dam about a castle he claimed to have out in the sea during a storm. If you go to the location which is way out to the east of the village of Arkos you will come to the castle which is the dungeon called Stormheim Castle. However, you must go to the location at a specific time otherwise all you will find nothing but the sea.
To make the castle appear in order to access it you must solve a simple puzzle. There is a clue when Harry in the tavern mentions he saw the castle during a storm. All you need to do is stand on the end of the dock and cast your Thunderstorm spell, doing this will make a castle emerge out of the water which you will be able to enter. This is not an easy dungeon to complete, so it is recommended that you stock up on potions before you tackle it.
Take a look at our How to get the Thunderstorm Spell guide to find out how you get this spell which is essential for getting Stormheim Castle to rise out of the sea.
We have questions and answers related to this topic which may also help you: