Tophets owe their existence to the King James spelling of a particularly grim region of Jerusalem (Wikipedia has the details). As constructs, these pot-bellied statues display an efficient and economical ruthlessness, being at once guardian, jailer, and executioner. PCs used to cutting their way out of acidic monster stomachs may find a conductive metal gut a bit harder to escape from.
In most adventures, tophets will act in the same manner as other constructs, coming to life to bludgeon PCs (with the added bonus of being able to swallow them as well). And you can expect most dungeon designers to take advantage of tophets’ fire resistance and construct nature as well, adding fire pits, burning braziers, lava streams, and even airless vacuums into the mix. Below I’ve tried to offer some more unexpected encounters…
A despotic king hires troublesome adventurers for a retrieval job: They must find his tophet. The king sent the tophet—its belly stuffed with a revolutionary leader—into the desert to bake its charge in the broiling heat. He has since discovered the condemned man concealed a powerful ring on his person. He needs the tophet back…but the construct has vanished, its tracks obscured by the sands. In fact, the iron glutton fell into the pit of a giant ant lion and has since been claimed by a rampage of particularly large hissing desert drakes.
Most adventurers would be mad to climb inside the maw of a tophet. But that’s just what thief and magus Cobbler Devish proposes. The Fiend Duke is expecting a shipment of the rotund furnace guards from the Sortilegeum, and anyone riding inside would bypass all his magical defenses…provided they could escape once the vessels are installed.
A tophet’s design is at the whim of its creator. Images of fat kings are common in the desert lands, while beyond the Drakewall roly-poly monks, wrestlers, and foo lions are more common. The witches of Hekkanov craft theirs to resemble ogres, and often boil victims inside as if the constructs were animate cauldrons. There is also at least one tribe of boggards that worship a bullfrog-shaped tophet of otherworldly origins like a god. This cruel idol seems to gulp its victims at random intervals, particularly when a comet hangs in the southern sky.
Pathfinder #21: The Jackal’s Price includes more historical background on tophets, plus some variants besides.