The wizard that discovered Prostiston was surprised to find that the island-sized slime mold was in fact an intelligent, spellcasting entity. Before Prostiton devoured him, his retainers --- fleeing rapidly --- heard him speculate that Protiston evolved slowly from ancient colonies of mold deep in the oceans, gradually developing intelligence and spellcasting ability. Other sages have speculated that Juiblex, the demon lord, had a hand (or pseudopod) in elevating Protiston to its current power.
But a few things are clear about Protiston: From all observations, it is completely incapable of recognizing sentience in other beings despite its own high intelligence. Protiston has no regard for other life, save its servitor macrobes, and views the entire world as its own to reshape or devour.
Protiston's not really island sized, there's nothing about it eating its discoverer, and I doubt very much it has any regard for its servitor microbes - they're just part of the world it has reshaped to serve it.
Hmm...
Protiston is a vast slime mold of enormous age which has developed terrible spellcasting powers and a formidable, albeit highly alien, intelligence. The mage who discovered and named Protiston claims that it slowly evolved from a simple colony of mold floating in some ancient sea. Other sages speculate that the demon Juiblex, the Faceless Lord, had a hand (or pseudopod) in Protistion's elevation.
Protiston is unable to comprehend even the possibility that anything other than itself might possess sentience. Everything it encounters is either food to be devoured, or a hostile environmental phenomena to be destroyed or reshaped to Protiston's benefit. Protiston wishes to reshape the entire world to benefit itself. It has the ability to transform ordinary microbial life into giant "macrobes" under its control. It has no more regard for the lives of these macrobes than it does for the rest of existence — they are nothing more than tools to feed its hunger and guard its treasure.
Protiston can move on land but lives in water. It may be found in the sea or freshwater lakes, most often lairing in coral atolls or caverns along the shoreline. Whatever place Protiston chooses as its home is only temporary, the sapient slime mold always moves on sooner or later. Sometimes this is because Protiston's devoured everything in its habitat, or it may have finished "reshaping" an area to its tastes and moved on to other project, or perhaps the chaotic creature decides to relocate on a whim. In adverse conditions, Protiston can turn part or all of its body into a sporocarp and wait for better times. Even the smallest piece of Protiston is part of the whole, the slime mold can regrow to full power even all of its body is destroyed apart from a single amoeboid cell.
Protiston can reshape the slime of its body in any shape it desires, it can even split itself into many parts and rejoin together again. Most commonly, it takes the form of a small island of slime up to 80 feet across and 20 or 30 feet thick. Protiston typically weighs between 1200 and 2000 tons.