Alambusha, AKA Shalakantaka, was a Rakshasa (demon) who fought on the Kaurava side in the Mahabharata war. As described in the Bhishma Parva of the Mahabharata, he was the one who killed Arjuna's son Iravan, and as described in the Drona Parva, he was eventually killed by Bhima's Rakshasa son Ghatotkacha.
But I'm interested in Alambusha's lineage. It is mentioned in the Mahabharata that Alumbusha is Rishyasringa's son. As I discuss in this question, Rishyasringa was the sage who performed the Putrakameshti Yagna for Dasharatha, which resulted in the birth of Rama and his brothers. And Rishyasringa was married to Dasharatha's daughter Shanta (who was raised by Romapada king of Anga).
So how is it that a son of the sage Rishyasringa and a grandson of Dasharatha was a Rakshasa? Did Rishyasringa have a Rakshasa son because of some kind of curse? Or did he marry a Rakshasi (demoness) after he married Shanta, similar to how the sage Vishravas married the demoness Kaikeshi (mother of Ravana) after he married Ilavida (mother of Kubera)?