1 Indrajit (MeghanAda) performed many sacrifices and got powers (boons, weapons, sidhis etc):

English Translation (Source)
O king ;thy son hath met with the fruits of many a sacrifice
Agnistoma, Asvamedha, Bahusuvarnaka. Rajasuya, Gomedha and Vaisnava. And being engaged in this Maheshvara sacrifice, which is incapable of being celebrated by men, thy son hath obtained boon from
Pasupati himself. He has also obtained illusory powers which create
darkness or ignorance namely of ranging in the sky, of being eternal,
of ascending a celestial car which curses at will and of creating
darkness.
O lord of Rakshasas, these illusory powers being usued in a conflict,
even the Devas and Asuras shall not be able to perceive his course.
Besides he has obtained a quiver, the arrows whereof shall never be
exhausted, a bow, which is hard of being got by and a dreadful weapon
which destroys enemies in a conflict.

English Translation (Source)
And having resorted to the illusory powers, conferred upon him in
yore by Pasupati he entered into the enemies camp and belaboured
them. Being invisible in the welkin by virtue of his illusory
powers and having brought Indra under the influence thereof he struck
him with hundreds of arrows. When Ravana's son came to know that Indra
was exhausted he, having bound him up by virtue of illusion,
proceeded towards his army. That conqueror of Sakra and subduer of
enemies, conversant with illusory powers, was not visible, by
whom, Indra, although master of many illusions, was carried away by
force.
2 Along with those weapons and boons he got siddhis and a boon of invincibility (in special conditions) from Lord Bhrahma:


English Translation (Source)
Thereupon the highly powerful Indrajit the subduer of enemies, said
"If dost thou say so, O god, I pray for immortality."Thereat the
highly powerfulpatriarch said to Meghanada "There is none perfectly
immortal on earth amongst created beings birds, quadrupeds and other
highly powerful beings." Hearing the words uttered by the grand father
the highly powerful Meghanada said to the eternal lord "Hear, then,
what I want in exchange for releasing Sakra. May a chariot with horses
rise up from fire when I shall offer sacrifices unto it, being
desirous of vanquishing my enemies in the conflict. And may I be
immortal as long as I shall remain on that car. This is the boon I
have resolved upon praying for. May I meet with destruction, O deity,
whenever I shall engage in fight without finishing my offerings unto
fire. All others, O god, attain to immortality by virtue of devout
penances but 1 shall acquire that by dint of my own prowess." Whereto
the grand father replied saying "so be it".