In the series finale Belos make his final gambit to destroy the Demon Realm for his heroic delusions as the witch hunter. But why he possessing the titan heart for?

When he was possessing Raine the Collector tell him about how powerful of the titans when they grow into massive, colossal sizes.

Which Belos realized it since King was the young titan, he will be defeated easier if he possesses the Collector since titans are the only creatures that countereffects against the Collector's kin. The titans are the most powerful beings of the Demon Realm because of their immense magical abilities Belos just found the only plot against witchkind by Becoming the titan.

Because of this Belos possess the titan heart at his old throne room and begin his progress by spreading his decaying flesh that kill anything on contact and result the Boiling Isles become his host body to wipe out the rest of the Demon Realm if it means to kill the Collector for splashing him back on the Day of Unity. But only his plan was stop by Luz in her titan form thanks to King's father give his remaining powers.