"Magical traps around madness"
The Maelstom appeared three generations ago, a by-product of a mad priest's attempt to speak directly to his deity. He contacted something immortal, to be sure. But not the intended, and not something amused to be disturbed by the incursions of some random mortal.
The immortal did communicate with the priest, in its fashion. Its alien intelligence pulled apart the priest's mind, stripping it as a flayed criminal at the stocks. Or more likely, as a bored youth pulling legs from a fly.
But intrigued by this new flavor, the intelligence reached farther afield into this primitive little world, seeking sensations. Fortunately, the mad priest's portal was flawed. Although the immortal mind could sense, its reach was limited.
But it could still entice...
The roiling portal drew the curious and the mad. Those who ventured within its sphere of influence found their minds shattered, their bodies left as starving, drooling husks in the wilderness.
Would-be rescuers met the same fate, as did the war-mages and shield-priests who attempted to interrupt and seal the breach. Any who approached the Maelstrom, no matter their wards and spells, collapsed under the mind's onslaught, leaving them as helpless as any fool.
Walls and patrols were proposed to keep the curious away. Walls were bypassed or breached. Patrols were evaded, or themselves strayed too close to the corrupted zone, and were lost to the hungry god-thing.
More arcane and impervious methods were required until a method to seal the portal was discovered.
First to keep the unwary away.
Then to make them forget.
At six miles, one felt an unease.
At five, fear took over, and bolted his mount.
At four, forgetfulness and disorientation turned him about - wandering away towards safety
If, for some reason, the wanderer, through force of will or other machinations, continued onward, at three miles, the amnesia became permanent. Brethren in enchanted helms would retrieve the lost, where they could be cared for until certain.. machinations... recreated them as useful beings again...