Aaju Dyaa also known as Aakash Bhairav and Yalamber is considered to be the first King of Nepal Mandal. Recognized for his Tantric powers was also the first Kirat King and is known for his contributions in battle of Kurukchetra in Mahabharata.
According to Hindu Mythology, he was given a wish by Lord Bharma the creator that whosoever he sides in any battle would never be defeated.
During Battle of Mahabharata, he sometimes resided the Karvars if they were about to lose and again the Pandavs if they were about to lose.
When Lord Krishna came to know this, he tricked him and chopped over his head which merely flew and fell at Wattu(modern day IndraChowk in Kathmandu), the exact spot where the temple stands.
The mask as in the picture is displayed once a year outside the temple during Indra Jatra festival.