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 underwent a cyberforming which turned its inhabitants into mindless drones who serve Unicron's will; Kranix, last survivor of Lithone, sometimes became this. Particular worlds rapidly mutated living beings on them into mindless beasts, such as on the planet Yst as Unicron utilized the planet to return to physical form. Others damaged the fabric of a reality which Unicron was not yet ready to consume—iterations of the Dark Nebula originally had such a planet at its heart. A few like Vestum Major became evil empires. There were additional types of tainted worlds, as Unicron's evil manifested 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.
• The list of tainted worlds given in Ask Vector Prime is based around those few worlds Unicron didn't devour. A connection to the G.I Joe vs. the Transformers incarnation of Unicron, and how he had left some parts of worlds alone, was cut from the final version.