The Skaro Degradations, or Skaro Devastation, were Dalek forces with abnormal and malformed appearances created during the Last Great Time War, which posed a greater threat than the original, unmodified species. The Degradations were created by the Daleks during experiments to retro-evolve the Dalek genome. The results created abominations of the Dalek form. The Daleks put the Degradations to use against the Time Lords during the War. Many were unstable and unpredictable. (PROSE: Engines of War)
They were described by the Tenth Doctor as one of the horrors of the Time War, along with the Horde of Travesties, and the Army of Meanwhiles and Neverweres led by the Could've Been King. He also cited an even more terrifyingly altered Dalek, the Nightmare Child. (TV: The End of Time)
"Gliders" was the name used by Cinder and her fellow resistance fighters on Moldox to identify the type of Degradation that travelled by hovering above the ground. The Kaled mutant, resembling a limbless humanoid torso, was housed in a reinforced glass casing suspended under a regular-looking Dalek dome. They had arms and energy weapons flanking them on either side. (PROSE: Engines of War)
The "Spider form" of the Fifth Skaro Devastation (AUDIO: The Neverwhen) was egg-shaped and moved on three spider-like legs. Their casing was red with black sense globes and they were armed with four energy weapons. (PROSE: Engines of War)
The Time Lords' attempt to interfere with the creation of the Daleks gave the Daleks the idea to conduct further experiments on their own race. They started entering as many alternative realities as they could and began tampering with both their history and their DNA to create a powerful weapon they could use against the Time Lords.
This ultimately resulted in the birth of the Skaro Degradations which were used by the Dalek Empire by the time of the Last Great Time War. They were used during the Dalek occupation of Moldox where the War Doctor encountered them. (PROSE: Engines of War)
When the Master broke the time-lock on the War as part of Rassilon's plan for the Ultimate Sanction, the Skaro Degradations were among the horrors of the War that threatened to break through. The Doctor and the Master were able to thwart Rassilon and re-seal the time-lock. (TV: The End of Time)