Mandrels were dangerous predators, able to easily kill a human. Mandrels are absurdly tough, and are able to survive repeated laser-blasts. The only known method to kill a Mandrel is to electrocute them. Luckily, they aren't especially intelligent and can be easily fooled.
They have the unusual characteristic that they decompose into Vraxoin after being electrocuted.
As part of a ritual to summon Artemis which required the blood of five species whose destinies had gotten affected by the Time Lords, the Monk included the blood of a Mandrel.
Before 2050,The Mandrels were one of the many races attacked by the Anubians.