The Filth was added in Eon Productions 7th expansion. It is a somewhat cumbersome power that tends to prolong the game.
You have the power to reek. When you land on a planet, all other tokens on that planet must leave and go to other bases. However, your allies in a winning offensive challenge are permitted to land on any other planet in the defensive system. No player can remain on a planet where you have tokens, and a player who somehow has a base there must immediately leave. When you agree to trade bases in a deal, you and that other player must each vacate a planet for the other player to land on. Players who win an offensive attack against your tokens on a planet do not immediately gain a base, but must return their tokens to their other bases. Your losing tokens go to the Warp normally and the planet is "fumigated". On subsequent challenges players can challenge the empty planet and move tokens onto it if they win.
Wild Flare - You may declare a planet "filthy" and all tokens there are returned to other bases (or to the Warp if they have no bases). Any tokens which should land there on that challenge return to other bases. Use once and discard.
Super Flare - When you are the offensive player and you point the cone at the defensive planet or moon, all other players' tokens there must immediately go to others of their bases (or to the Warp if none).
The Filth's reverence for the past leaves it unable to remove any trace of earlier events - whether yesterday's glatorp stains or last year's diseased kinzosh. Having driven all other lifeforms from their home worlds, they are slowly extending out to leave their mark - a ring around the Cosmos.
The main benefit the Filth get for their power is that it takes two challenges for enemy players to conquer a Filth planets - one to kill the defenders, then a later challenge to actually land troops on the world. This means that they generally lose planets at only half the rate of other players.
A disadvantage of the Filth is that other players are more reluctant to consummate a deal, since it means they actually have to evacuate the planet.
Another characteristic of the Filth is that they tend to destroy other players' bases over time. For instance, if the Vulch has tokens on one of the Mind's planets, and the Filth successfully invades that planet, the Vulch must evacuate as well as the Mind, thus losing one of his bases. This makes the Filth more and more disliked over the course of play. Which probably makes sense.