Ruru is the un-captured life energy of the sacrificial victim, after somayagya is used to captures its life force. The Somyagya worked on the principle of PanchDhatu. It captured the fifth element, that is, the life force and the rest elements- prithvi, agni, jal, vayu were left as byproducts, not captured by the technique discovered by Tvastr.
This ruru energy was intelligible, confused and roamed imperceptible to the common eye. Only a few could feel its presence, most notably Yama and Shani. This could be because both of them were responsible for sacrificing and disposing the body of the victim.
As they were only elemental life energies, they could reside in places where life could not. They were attracted to heat and were found mostly in desert, where they could not be detected easily and where heat was extreme. During the 10 wars, the Devas conducted 1000s of somyagyas and thus, it is believed that many such rurus exist in the world, esp. in the region of naraka.
Some rurus, could take over the living beings, esp. if they could subdue the lifeforce, prana, of the host body. It was rare previously, but the incidences of their sightings had increased recently. Many traders were forced to keep mercenaries with their troop to protect themselves of such dangers.
Pishachas abilities were not known completely, but some of them were extremely powerful. They were violent and had basic emotions. Some of them though, had high intelligence like syam and sabal.
Due to their powers, many mercenary groups had resorted to hunting these Pishachas, esp. Taksak's army. Some of the mayavi's had discovered that a new element known as krsn-ayas could be made stronger and more powerful with the blood of ruru. Taksak's army on the other hand were known to frequently cook pishachas meat and drink their blood to get access to occult powers. But all this was not without its side effects.
Some Pishachas could retain their basic form and change to their paishachi form at will, most notably Yama himself. With the help of Shani, he could also transfer the rurus residing in him to other Yamadutas, which is seen once in the battle with mara and mahisha.
Pishachas had to eat to keep their host body alive. But some of these pishachas, resorted to eating other pishachas to the ruru in them. These rurus, who ate other rurus to become stronger were known as Rakshasas. Such beasts were very few, but whatever were there, were extremely powerful and dangerous.
Yama was more of a rakshasa as he fed on other rurus to make himself stronger. He could not eat every ruru, as some of them were too powerful for him to control.