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The xXltttxtolxtol were barely humanoid, with tendrils for limbs and no apparent faces. They had knobbly, bark-like skin. They secreted a sticky sap which could be broken down with alcohol, which also sickened the xXltttxtolxtol as well. Though they had no visible mouths, they had deep rumbling voices. (PROSE: Risk Assessment)
The xXltttxtolxtol created space probes which contained a faster-than-light drive, instructions on how to build a spaceship around it and the location of their home planet. They also used metal pods for transport in the Cardiff Space-Time Rift. These pods were nearly impenetrable and only transmitted trace energy signatures. When configured in a particular way, they could be used to fix a time rift and use it as a portal. When the rift was being fixed, the xXltttxtolxtol had to stand in certain places and would be rooted in place until it was finished.
In the 20th century, the xXltttxtolxtol planet was dying and had a few thousand years left. The xXltttxtolxtol sent out probes to find a new planet to live on. Earth took the bait and George Herbert Sanderson travelled to Planet X. When they learned of the Cardiff Space-Time Rift, they developed a way to use the rift to quickly transport George's ship and a group of life pods back to Earth. Agnes Havisham helped them find a way to hide their plan from the Torchwood Institute, believing that they would interfere and not realising that the xXltttxtolxtol planned to invade. Nearly a hundred pods were sent through the rift, disguised as coffins. Coincidentally, a fragment of the Vam had attached itself to the pods and started growing on Earth.
After the Vam was defeated, Agnes activated the pods. It was only when the xXltttxtolxtol revealed their plan that Agnes realised their true purpose on Earth. They bound Agnes, Jack and Ianto, shot down George's ship and started turning the pods into a portal to Planet X. Taking advantage of the crash, Gwen Cooper (who had been brought to the ship) called Jack with her mobile phone. Using Jack's phone as a homing signal, George and Gwen aimed the ship at the portal, then escaped the ship. Jack, Agnes and Ianto escaped using the rum Agnes carried to break down the sap binding them. When the ship crashed, it lit the remains of the Vam on fire, utterly destroying the xXltttxtolxtol. (PROSE: Risk Assessment)