1) Lord Shiva appeared as a yaksha to teach the proud devas a lesson after they defeated the asuras without recognizing Shiva's role. 2) Agni, Vayu, and Indra were unable to determine the yaksha's identity through their powers of burning, moving objects, and strength. 3) Indra then saw Uma, who was doing penance to marry Shiva, and she revealed that the great spirit was Shiva, the absolute Brahman. The devas then understood Shiva's supreme nature.

1. Lord Shiva --- A story from Kena Upanishad and Shiva maha Purana.
Yaksha Swaroopaaya Jataadharaaya
Meaning: Prostrartions to the sacred Lord who is the Yaksa
incarnate, whose hairs are long and matted, who holds Pinaaka
(trident) in His hand, who has the entire sky as His attire and who is
embodied as the letter 'Ya'.
===== OM Namah Shivaya =======
After Khseera sagara madanam, the Devas got Amrutam. The devas
became very strong and easily defeated the Asuras. The Devas
thought that it was their own strength which was responsible for the
Victory over Asuras. They forgot altogether that it was only by the
grace of Lord Shiva,the absolute brahman. The Devas started feeling
very proud about their strength. They said to themselves :
"vijayo'smakamevayam mahimeti " ( ie., "Verily, this victory is ours;
verily, this glory is ours only.")
Lord Shiva ,the omniscient, decided to teach the Devas a lesson. So
he appeared before Devas in form of an Yaksha. The Gods decided
to know about this new creature.

2. Lord Shiva In the form of Yaksheswara
The devas asked AgniDeva : "O Agni! Find out who this great Spirit
is."

3. Agni deva :"Yes,"
Agni Deva
Lord Shiva who's in Yaksha form, asked him: "Who are you?"
Agni deva said : "I am known as Agni; I am also called Jataveda."
Yaksha said: "What power is in you, who are so well known?"
Agni deva replied: "I can burn all— whatever there is on earth."

4. Iswara put a straw before him and said: "Burn this."
Agnideva rushed toward it with all his ardour but could not burn it.
Then he returned from the Spirit and said to the gods: "I could not
find out who this Spirit is,"
Then the devas said to Vayu : "O Vayu! Find out who this great
Spirit is."
Vayu deva said : "Yes," , then he hastened to It.
Vayu Deva

5. Parameswara asked him: "Who are you?"
Vayu replied : "I am known as Vayu; I am also called Matarisva."
Lord shiva said: "What power is in you, who are so well known?"
Vayu replied: "I can carry off all— whatever there is on earth."
Brahmanputa straw before him and said: "Carry this." He rushed
toward it with all his ardour but could not move it. Then he returned
from the Spirit and said to the gods: "I could not find out who this
Spirit is,"
Then the gods said to Indra: "O Maghavan! Find out who this great
Spirit is."

6. Indra said: "Yes," and hastened to It.
Indra
sa tasminnevakashe striyamajagama bahushobhamanamuma
haimavatim tamhovaca kimetadyakshamiti
But the Lord shiva disappeared from him. Then Indra beheld in that
very region of the sky a Woman highly adorned. She was Uma, the
daughter of the Himalayas. In order to marry Lord shiva, she was

7. doing severe penance. Uma knew the supreme nature of Lord Shiva.
Uma devi, The Daughter of Himalayas
Indra approached her and said: "Who is this great Spirit?"
Parvatireplied : "It is, indeed,Absolute Brahman Shiva . Through the
victory of Brahman alone have you attained glory." Later, she taught
Indra about the supreme nature of Lord Shiva. After that Indra
understood that It was Brahman(Lord Shiva).

8. Since they approached very near in comprehending the Supreme
nature of Lord Shiva and were the first to know that the Yaksha was
Brahman(shiva) , these devas, namely, Agni, Vayu and Indra,
excelled the other gods.