The mrdanga is a traditional Indian percussion instrument used to accompany bhajans and kirtans.
It is said that whenever mrdanga and karatals are combined, a mohini shakti (potency of enchantment) is produced that creates an auspicious atmosphere. The following story describes the appearance of the mrdanga in the pastimes of Caitanya Mahaprabhu:

“Before coming to this world as the Golden Avatara, Lord Krishna said to his flute, “My dear flute, I am descending again to the material world but not in this form. You should stay here with Radharani to pacify Her. I promised Radha that I would not take you with me”. On hearing this, the flute began to cry and said, “My dear Lord Krishna, how can You leave me here alone. I have always been by Your side, but now You are neglecting me. What have I done to deserve this?”

Krishna replied compassionately that he would bring him along but in a different form. The flute did not mind changing his form, as long as he could remain engaged in Krishna’s personal service, in this way the flute transformed into a mrdanga, and descended to this world with Sri Caitanya Mahaprabhu.
The mrdanga is not an ordinary musical instrument. Being the incarnation of Krishna’s flute it is very dear to Lord Krishna.” (Mahanidhi Swami, Appreciating Navadvipa Dhama)

Whenever mrdanga and karatals are combined, a mohini shakti is produced