How did Brahman/Yaksha break Indra's arrogance?

 Brahman (here portrayed as Shiva) breaking the Pride of the devas is described in Kena Upanishad.

Devas became arrogant because of their victories disregarding the fact that they obtained it only due to Brahman. Brahman therefore appeared before them to test them

The Brahman won a victory for the Devas and in that victory of the Brahman the Devas attained glory. They thought ‘the victory is ours and this glory is ours alone.’ He (Brahman) knew this notion of theirs and appeared before them. What that Great Spirit was they did not know.

• Verses 14–15, part 3, Kena Upanishad.

The devas sent Agni to find out who this person was. Brahman put forward a straw and asked Agni to burn it if he can. Agni tried but failed miserably

They addressed the Fire thus “O Jataveda! Find out what this Great Spirit is.” He said “yes.” He ran to That. That said to him “who art thou?” He replied “I am Agni or I am Jataveda.” That said “what power, in thee so named, is lodged.’ He replied “I can burn even all this, on the earth.” That (Brahman) placed a straw before him and said: ‘Burn this.’ He approached it with all haste but was not able to burn it. He immediately returned from thence to the Devas and said I was not able to learn what this Great Spirit is.”

• Verses 16–19, part 3, Kena Upanishad.

The devas sent Vayu to find out who this person was. Brahman put forward a straw and asked Vayu to blow it away if he can. Vayu tried but failed miserably

The Devas then said to Vayu: “Learn O Vayu! what this Great Spirit is” He said: “yes.” He ran to That. That said: “who art thou”? lie replied: “I am Vayu or Matarisva.” That said “what power is in thee; so well known?” He replied: “I can blow away all the universe and all that is on the earth.” That (Brahman) placed a straw before him and said “Blow it away.” He approached it with all speed but was not able to blow it. He returned immediately from there and told the Devas “I was not able to learn who this Great Spirit is.”

• Verses 20–23, part 3, Kena Upanishad.

Finally, the devas send Indra to find out the person’s identity. As he approached, Indra saw the great Uma Parvati beside the man. She revealed that the man was Brahman himself

Then they said to Indra: “Maghavan! learn what this Great Spirit is.” He said “yes” and ran to That. That vanished from his view. He beheld in that very spot a woman, Uma, very beautiful and of golden hue, daughter of Himavat. He said to her “What is this Great Spirit?” She said “It is Brahman indeed. Attain glory in the victory of Brahman.” From her words only, he learned that it was Brahman.

• Verses and 24–26, Part 3 and 4, Kena Upanishad.

Because Indra realised the Brahman, he became the best among the devas

These Devas Agni, Vayu and Indra therefore much excel others, because they touched the Brahman nearest. They it was who first knew the Spirit to be Brahman. Therefore also does Indra considerably excel other Devas because he approached Brahman nearest and because he first knew the Spirit to be Brahman.

• Verses 27–28, part 4, Kena Upanishad.

What was the significant boon given by Indra to King Kuru, and how did it affect battles in Kurukshetra?

 

The boon that Indra gave to King Kuru that anyone dying on the land of Kurukshetra would attain heaven regardless of their deeds had psychological implications during the Kurukshetra war,

“The very dust of Kurukshetra, borne away by the wind, shall cleanse persons of wicked acts and bear them to heaven!" The foremost ones amongst the gods, as also those amongst the Brahmanas, and many foremost ones among the kings of the Earth such as Nriga and others, having performed costly sacrifices here, after abandoning their bodies, proceeded to heaven. The space between the Tarantuka and the Arantuka and the lakes of Rama and Shamachakra, is known as Kurukshetra. Samantapanchaka is called the northern (sacrificial) altar of Brahman, the Lord of all creatures. Auspicious and highly sacred and much regarded by the denizens of heaven is this spot that possesses all attributes. It is for this that Kshatriyas slain in battle here obtain sacred regions of eternal blessedness. Even this was said by Shakra himself about the high blessedness of Kurukshetra. All that Shakra said was again approved and sanctioned by Brahman, by Vishnu, and by Maheshvara.'"

Shalya Parva Section 53 K M Ganguli’s Mahabharat

It made Kurukshetra a sacred battlefield, deflecting it from a dynastic conflict.

Arrogant and power-hungry, in his final moments, lying mortally wounded, Duryodhan declares he would go to heaven and mock the Pandavas. This reflects a belief in heavenly reward despite his actions.

Karna deeply believed in Dharma. He knew he had sinned (e.g., in Draupadi’s humiliation).

Kurukshetra being a holy land gave him hope that death would redeem him.

King Kuru’s boon, the safety net it provided shaped the ethos of Kurukshetra war — where even the worst of sinners could die as warriors and ascend.

Drona and Bhishma fighting on the side of Evil (the Kauravas) despite knowing the moral truth but deeply tied to their sense of duty and personal vows, were possibly aware of Kurukshetra’s spiritual boon.

The deaths of valiant Kshatriyas at the battle of Kurukshetra with the boon transforms the tragedy into a spiritually sacrifice.

picture from online source