Tainted world

A "tainted world" is a world created (or possibly re-created) by the chaos-bringer Unicron where everything you know is wrong. Black is white, up is down, short is long, cats chase dogs, and Vegemite is a snack food.

Unicron began to consume world after world, reabsorbing the energy that it had lost creating other tainted worldsThe Ultimate Guide

Fiction

Ask Vector Prime

As Vector Prime explained, some tainted worlds undergo a cyberforming which turns its inhabitants into mindless drones who serve Unicron's will. Sometimes Kranix, last survivor of Lithone, becomes this.

Some tainted worlds rapidly mutate living beings on them into mindless beasts. This occurred on the planet Yst as Unicron utilized the planet to return to physical form.

Some tainted worlds damage the fabric of a reality which Unicron is not yet ready to consume. Some iterations of the Dark Nebula originally had such a planet at its heart.

Other tainted worlds are such that their populations become evil empires, such as Vestum Major.

And of course, there are other kinds of tainted worlds. Unicron's evil, after all, manifests in countless ways. Ask Vector Prime, 2015/08/09

Notes

• The phrasing in The Ultimate Guide suggests that Unicron created these 'tainted worlds' prior to his imprisonment by Primus and assumption of his current mechanical form. Whether this would actually hold in practice is debatable.
• Tainted worlds have only been seen in fiction in the Facebook version of Ask Vector Prime, ten years after they were first mentioned in The Ultimate Guide.
• The list of tainted worlds given in Ask Vector Prime is based around those few worlds Unicron didn't devour. There also was originally intended to be a connection to the G.I Joe vs. the Transformers incarnation of Unicron, and how he had left some parts of worlds alone, but this was cut from the final version.
• The strange technorganic world depicted in the Transformers: Armada episode "Puppet" seems to bear some similarities to the idea of a world tainted and controlled by Unicron: its only inhabitant is his bloodthirsty servant Nemesis Prime, is accessed by a portal specifically created by the Chaos Bringer's agents, and begins disintegrating after Nemesis' destruction (seemingly for no other reason then as a last attempt to kill the assembled Transformers).