This slayer boss works a bit differently from others - upon killing enough ghosts to spawn the boss, it spawns a portal to a separate instance, kinda like the glacite mineshafts. This is the misty abyss, a special arena to fight the slayer boss in. Upon defeating the boss, you can exit the misty abyss back to the dwarven mines lobby you were in before, spawning in the middle of the mist.
The misty abyss is a circular platform made up of ice that melts as time goes on. Falling in the water will deal 20% of your health as true damage per second and also cause you to rapidly accumulate cold. Speaking of cold, your cold level will also increase while in the misty abyss, being slowed down by cold resistance of course. This, along with the melting ice, provides a natural timer to the boss fight, resulting in an interesting challenge for the player!