SB 4.14.42
nangasya vamso rajarser
esa samsthatum arhati
amogha-virya hi nrpa
vamse ’smin kesavasrayah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The sages decided that the descendants of the family of the saintly King Anga should not be stopped, for in this family the semen was very powerful and the children were prone to become devotees of the Lord.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The purity of hereditary succession is called amogha-virya. The pious seminal succession in the twice-born families of the brahmanas and ksatriyas especially, as well as in the families of vaisyas also, must be kept very pure by the observation of the purificatory processes beginning with garbhadhana-samskara, which is observed before conceiving a child. Unless this purificatory process is strictly observed, especially by the brahmanas, the family descendants become impure, and gradually sinful activities become visible in the family. Maharaja Anga was very pure because of the purification of semen in the family of Maharaja Dhruva. However, his semen became contaminated in association with his wife, Sunitha, who happened to be the daughter of death personified. Because of this polluted semen, King Vena was produced. This was a catastrophe in the family of Dhruva Maharaja. All the saintly persons and sages considered this point, and they decided to take action in this matter, as described in the following verses.