Tainted world

Tainted worlds are spread across the multiverse and may or may not exist in any continuity Unicron has visited.

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 worlds, Transformers: The Ultimate Guide, p50

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, 2015/09/22

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.
• None of these tainted worlds have ever been seen or encountered, but they have to exist somewhere in the Multiverse. In 2015 the Facebook version of Ask Vector Prime finally explained and gave examples of tainted worlds.