Padmasambhava received the siddhi of the kīla transmission from a gigantic scorpion at the charnel ground of Rajgriha:
The sting of the scorpion's whip-like tail transfixes and poisons its prey, and in this respect it is identified with the wrathful activity of the ritual dagger or kīla. Padmasambhava's biography relates how he received the siddhi of the kīla transmission at the great charnel ground of Rajgriha from a gigantic scorpion with nine heads, eighteen pincers and twenty-seven eyes.
This scorpion reveals the kīla texts from a triangular stone box hidden beneath a rock in the cemetery. As Padmasambhava reads this terma text spontaneous understanding arises, and the heads, pincers, and eyes of the scorpion are 'revealed' as different vehicles or yanas of spiritual attainment.
Here, at Rajgriha, Padmasambhava is given the title of 'the scorpion guru', and in one of his eight forms as Guru Dragpo or Pema Drago ('wrathful lotus'), he is depicted with a scorpion in his left hand. As an emblem of the wrathful kīla transmission the image of the scorpion took on a strong symbolic meaning in the early development of the Nyingma or 'ancient school' of Tibetan Buddhism...".