In a parallel universe, the Quatch were an alien race who had a vast empire in the same region of space as Skaro. They had a long history with the planet, having engaged the Thals in a battle in orbit of Skaro at somepoint in the past. Skaro itself was desired due to the vast reservoir of dimensional energy at its core.
The Quatch were able to predict and anticipate the actions that Skaro's natives would take when engaged in a full out war and did this to many of the species that they destroyed and enslaved.
After this, they provoked existing animosity between the Kaleds and the Thals, resulting in all out war between the two.
The Quatch were not native to this universe. Their own ethereal bodies required complex biomechanical augmentations simply to anchor them in this reality. They encountered Davros sometime after the war's end and persuaded him to abandon a stagnant Skaro and a Dalek race which had failed to meet Davros' expectations.
During their partnership, Davros developed and improved Quatch technology, including the parts which formed their anchor. In respect for his contributions, they allowed Davros to recruit whichever Daleks would still follow him, but only by re-engineering them to be loyal to the Quatch.
They were expelled from reality when the Doctor infiltrated their mothership and revealed the Quatch's machinations over the state of Skaro. A defiant Davros utilised a back door installed in their mechanical components, causing them to painfully fall back into their home universe. (AUDIO: Masters of War)