Sure, black puddings are bad. But at least they're not on fire and thrusting a sword-like shard into your gut.
According to Bestiary 4, shard slags “dwell within the iron-rich molten cores of worlds, the hearts of active volcanoes, or any location where the borders of the Material Plane and the Plane of Fire are thin.” I especially like #1 in that series—I’m a sucker for center-of-the-world adventures.
But you don't have to go that far to look for a shard slag. With more and more campaigns flirting with steampunk/magitechnical levels of industry, adventurers might be only a few blocks away from a potential shard slag eruption.
The Plane of Fire exhibit in the Gymnasium Zoologikum has an intruder! A shard slag has managed to pull itself through the pinhole leak to the Fire Plane that warms the exhibit. The fire, steam, and magma mephits have fled to the highest corners of the enclosure (and the poor magmins have already been consumed). Adventurers who answer the call for help must fight the ooze in tight quarters…and ideally determine whether its presence is an accident or sabotage.
When the foundry on the Street of Shedus explodes, the resulting torrent of magical energy and molten hot iron manages to transform the bound fire elementals that heated the furnaces into something far more deadly—a supersized shard slag. Even before adventurers put down the slag and put out the fires, they find themselves under attack—first by the foundry’s private guards and then members of the city’s own wand-wielding fire brigade. Whatever burned in the fire, someone important has something to hide.
Adventures flee from morlock vampires and find themselves lost in a series of great chambers below the earth. One superheated chamber hides a shard slag intent on consuming them. The sound of lapping waves offers salvation—a bay that opens into a shallow, sauna-like sea—and tactically minded adventurers might use the presence of water to escape the hydrophobic ooze. Of course, then they have to face the bay’s resident gargiya (see Pathfinder Adventure Path #60: From Hell’s Heart), a sea monster that flourishes in such steamy waters.
—Pathfinder Bestiary 4 240
You can find a shard slag in wesschneider’s level of The Emerald Spire Superdungeon, a veritable playground of things hoping to burn you, vivisect you, or both.
What the hell, number three.