If your opponent destroys your Prison Realm, they get their boyo back.
I don't know whether Nicol Bolas is now a creature or Planeswalker, but it's rather immaterial since he's locked up in the Prison Realm. I would bet his card type would be "Sad, lonely bastard who has all of eternity to contemplate how badly he ****ed up"
Since you could just read the card The Immortal Sun, I assume you're asking about the lore. The Immortal Sun is a magical artifact that prevents Planeswalkers from Planeswalking away. Bolas used it to trap Planeswalkers on Ravnica so he could kill them all without the risk of them running away. He didn't care about their abilities because he was such a cocky prick he figured they wouldn't be a challenge. Below is the description about Bolas's usage of the Immortal Sun during the War of the Spark, from https://mtg.gamepedia.com/Immortal_Sun
Bolas used the Immortal Sun to trap many planeswalkers on Ravnica, after they had been lured there by the Interplanar Beacon, to harvest their sparks. Chandra, Nissa, Saheeli, and Lavinia (plus Lazav) were tasked with shutting off the Immortal Sun. At New Prahv, they managed to do so after defeating Dovin Baan, which resulted in a lot of planeswalkers fleeing the plane. Jace had taught Chandra Azor's nine words that would shut off the Sun, that he had dug up from the sphinx's mind. Chandra managed to shut it off with the spell after briefly being kind of possessed by its power.
Afterwards, before the final battle with Bolas, they decided to reactivate the Immortal Sun to prevent Bolas from escaping. The Immortal Sun was shut off again at the very end after Bolas was defeated, but it was not destroyed in the event that it's needed again.
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"Life is a jape. Yours, mine, everyone's."
—Tyrion Lannister, A Dance with Dragons