Actually, that was bad wording on my part: Mechanus is the Plane of Law, where the Inevitables are from. Just like Baator has Devils and the Seven Mounting Heavens has Archons, Mechanus has Inevitables.

These clockwork Outsiders enforce the laws of the multiverse, and each has a specific function. A Marut, for example, is sent after those who cheat death an extraordinary number of times. A Siphonut, it seems, gets sent after anything another Inevitable fails to bring to justice. The wording of the Summoner's Poison seems to indicate that transgressors are not always brought to Mechanus... which is an interesting possibility I had never considered before. (Hey, here's a novel idea! I'll link the SRD!)
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A question, however, regarding the wording of Summoner's Poison: Is it supposed to be overcomable by magic, or not? It says it can, but then the wording shifts as if it's not... I am confuzzled.