I can recall that I read in a question that Prahalad once attacked Devaloka and maybe won over Indra. I wanted to know if it is true or not and if true, then what is the story behind this and why did Prahalad, being a devotee of Lord Vishnu and good natured, attack the Devatas? Also, whether the Tridevs applied any trick/battle with him to help Indra get back his throne.
Yes, Prahlada did conquer Devaloka. People don't generally discuss Prahlada's adult life, because it's not nearly as inspiring as the story of his childhood. He maintained his devotion to Vishnu and his good character, considering that he instilled both in his grandson Mahabali, but like Mahabali he seems to have had a desire to have the Asuras take over the three worlds, perhaps out of loyalty to his own Asura race. And he succeeded, as described in this chapter of the Shanti Parva of the Mahabharata:
In days of yore, the Daitya Prahlada, by the merit of his behaviour, snatched from the high-souled Indra his sovereignty and reduced the three worlds to subjection.
In response, Indra disguised himself as a Brahmin and approached Prahlada, asking him how he managed to conquer the three worlds. Prahlada responded that it was because of his virtuous behavior:
I do not, O regenerate one, feel any pride in consequence of my being a king, nor do I cherish any hostile feelings towards the Brahmanas. On the other hand, I accept and follow the counsels of policy they declare unto Me based upon the teachings of Sukra. In complete trustfulness they say unto me what they wish to say, and restrain me from courses that are unrighteous or improper. I am ever obedient to the teachings of Sukra. I wait upon and serve the Brahmanas and my seniors. I bear no malice. I am of righteous soul. I have conquered wrath. I am self-restrained, and all my senses are under my control. These regenerate ones that are my instructors pour beneficial instructions upon me like bees dropping honey into the cells of their comb. I taste the nectar dropped by those learned men, and like the Moon among the constellations I live among the members of my race. Even this is nectar on earth, even this is the clearest eye, viz., listening to the teaching of Sukra from the lips of Brahmanas and acting according to them. In these consists the good of a man.
Prahlada was impressed that the Brahmana was patiently listening to all this, so he offered him a boon. Indra asked Prahlada to give him his good behavior. Prahlada agreed, and his good behavior left him in the form of a god. After that, gods associated with righteousness, truth, good deeds, might, and prosperity all left Prahlada and entered Indra. Thus rule of the three worlds was restored to Indra and the gods.
By the way, this isn't the only occasion where Indra approached Prahlada for wisdom; this later chapter of the Shanti Parva of the Mahabharata describes another such occasion, and it's effusive in its praise of Prahlada:
The chief of the Daityas, viz., Prahlada, was unattached to all worldly objects. His sins had been washed away. Of respectable parentage, he was possessed of great learning. Free from stupefaction and pride, ever observant of the quality of goodness, and devoted to various vows, he took praise and censure equally. Possessed of self-restraint, he was then passing his time in an empty chamber. Conversant with the origin and the destruction of all created objects, mobile and immobile, he was never angry with things that displeased him and never rejoiced at the accession of objects that were agreeable. He cast an equal eye upon gold and a clump of earth. Steadily engaged in study of the Soul and in acquiring Emancipation, and firm in knowledge, he had arrived at fixed conclusions in respect of truth. Acquainted with what is supreme and what is not so among all things, omniscient and of universal sight, as he was seated one day in a solitary chamber with his senses under complete control.
Another notable incident in Prahlada's adult life is when he fought the sages Nara and Narayana, not knowing they were incarnations of Vishnu, because he thought they were violating dharma by carrying weapons, as described in this chapter of the Devi Bhagavatam.
In any case, Prahlada ultimately entered Patalaloka with his grandson Mahabali, as described in this chapter of the Srimad Bhagavatam, and he later attained Moksha.
An encounter of Prahlada and Indra did happen.
And the encounter that took place between that monkey and the Rakshasa hero, is desirous of defeating the other, was fierce and terrible, like that of Indra and Prahlada (in days of yore).