I was thinking that Ahiravan and Mahiravan are two different Rakhasas but according to the answer of this question, I found both are two different names of same Rakhasa and he is a brother of Ravan. I never listened how he died.
Was he killed in the Ramayana war or somewhere else? Who killed him?
As per 7th verses of Sankatmochan Hanumanashtak
बंधु समेत जबै अहिरावन लै रघुनाथ पताल सिधारो।
देबिहिं पूजि भली बिधि सों बलि देउ सबै मिलि मंत्र बिचारो।
जाय सहाय भयो तब ही अहिरावन सैन्य समेत सँहारो।
को नहिं जानत है जग में कपि संकटमोचन नाम तिहारो॥७॥
English Translation
When Ahiravan secretly carried away Lord Rama and his brother Laxmana to the ‘patal-loka’ (nether-world), the demon consulted his friends and decided to sacrifice the life of the two divine brothers at the altar of his chosen Deity with due ceremony. But O Hanuman, it was you who reached there for the rescue of your Lord Rama and his brother, saving their life by killing Ahiravan and his army. O Hanuman! Who does not know in the world that you name dispels all stress! (Source)
Footnotes
Sankatmochan Hanumanashtak (संकटमोचन हनुमानाष्टक), literally Eight verses for Hanuman, the Remover of Afflictions, is an Awadhi work of eight verses in the Mattagajendra metre, devoted to Hanuman. It is believed to have been composed by Tulsidas on the occasion of the founding of the Sankatmochan Temple in Varanasi. The work is usually published along with Hanuman Chalisa. (Source)
The story of Airavan and Mahiravan abducting Rama and Lakshmana is found in Ananda Ramayana. They are two different persons.
I have written answer to Who were Ahiravan and Mahiravan? .
The source of legend of Airavan and Mahiravan is Ananda Ramayana. It is part of story of Rama which is spread in vast expanse (Charitam Raghunatasya Shatakoti Pravistaram). According to that text, both the Rakshasas were slain by Lord Rama.
Hanuman devised a plan to remove the immortality of Ahiravan and Mahiravan by destroying the honey bees which were providing nectar to Airavan and Mahiravan's blood. I explained that story in detail in my other answer.
tato nihatya tau daityau punarbāṇai raghūdvaḥ।
amiṣicya tayo ḥ sthāne rājye taṃ makaradhvajam ।।Rama killed those two daityas using arrows and consecrated Makaradhvaja as ruler. (Verse 122, Sarga 11, Sara Kanda)
Answer is based on Hindi translation of Ananda Ramayana by Ramtej Pandey.
