Trimurthi is the trinity of three deities Lord Brahma, Lord Vishnu and Lord Shiva. Ekapada Trimurthi is the form of Lord Shiva where he is described along with Lord Brahma and Lord Vishnu forming the trinity.
In Ekapada Trimurthi aspect, the central leg of Shiva is more visible than other two smaller legs of Vishnu and Brahma, who emerge from the sides.
As it is described in the Linga Purana, Lord Shiva is the lord with one foot, four arms, three eyes and a trident and he stationed after creating Vishnu from his left side and four-headed Brahma from his right side. As it is well known, the Lord Brahma is creator, Lord Shiva is the destroyer and Lord Vishnu looks after all the Beings in the Universe.
The existence of Ekapada Timurthi has been discussed in some of the epics but it is not sure when the exact occurrence of this form of Lord Shiva has taken place. It is also believed that the Ekapada Trimurthi is the most uncommon form of all the Hindu deities and it is known for its versatility and power.
Since all the three Lords are formed to have one unique power and the power is controlled by Lord Shiva, Ekapada Timurthi is considered to be the most powerful form of Lord Shiva by the Shaivites.
The icons of Ekapada Trimurthi are mostly found in the temples of South India and Orissa. In some temples the icons of Ekapada Trimurthi are along with some popular sages and other Gods.