You are, all of you, vermin! Cowering in the dirt, thinking... what, I wonder? That you might escape the coming fire? No! Your world will burn until its surface is but glass! And not even your Demon will live to creep - blackened - from its hole to mar the reflection of our passage, the culmination of our Journey. For your destruction is the will of the gods! And I? I am their instrument!
The Prophet of Truth first appeared during Supreme Commander Thel Vadamee's trial, where he was already seen as more dominant than the other Hierarchs and later made Thel the new Arbiter of the Covenant, making his first assignment to assassinate Sesa Refumee and his Heretic followers. After the recent murder of the High Prophet of Regret, Truth organized the Changing of the Guard where Elites would give up their positions as Honor Guards to the Brutes, one of the stages that Truth planned to overthrow the Elites with Brutes.
After the capture of 343 Guilty Spark, he sent Thel Vadamee to retrieve the Index (Sacred Icon) from the Flood and Forerunner Sentinels that are guarding it. Truth soon had the Index in his hands and when John 117 infiltrated High Charity, Truth escaped alongside Mercy and ordered his Brutes to kill him. John's main objective was to find Truth and get the Index from him, but Truth made his way to the Phantoms with Mercy and Tartarus. He gave the Index to Tartarus and wanted him to begin the Great Journey, then Truth left High Charity to lead Covenant forces in invading Earth.
On Earth, Truth led a fleet and first appeared in Crow's Nest where he gave a hint to the UNSC forces there that they will die in the fire. Holograms of Truth was then spreaded throughout various missions that involved Covenant enemies, where Covenant forces preached to him and listened to his every word until an enemy attacks them. Truth then appears in person on The Covenant, where he has Sergeant Johnson captured and used him to fire the Halo Array, he also murdered Miranda Keyes with a Brute Spiker. Both John 117 and Thel then confronted Truth and stopped him froming activating the rings. Thel then had his revenge by impaling the Prophet, ending Truth once and for all.
Halo 3: ODST
Prophet of Truth assembled a Brute fleet to attack the New Mombasa which ODSTs were deployed in and were ordered to kill all the remaining Elite commanders as well as Human survivors. If you get the Legendary Ending, Truth will appear in person with two Engineers beside him and oversee Phantoms going to Earth.
Halo's destruction was your error and you rightly bear the blame. But the Council was... overzealous. We know you are no heretic. This is the true face of heresy, one who would subvert our faith and incite rebellion against the High Council.
~ Truth in the mission The Arbiter
The release of the parasite was unexpected, unfortunate. But, there is no need to panic. In truth, this is a time to rejoice. A moment that all the Covenant should savior. For the Sacred Icon has been found. With it, our path is clear, our entry into the Divine Beyond guaranteed! The Great Journey is nigh...and nothing, not even the Flood, can stop it.
~ Truth in the mission Gravemind
Shall we let the Flood consume our holy city? Turn High Charity into another of their wretched hives? No enemy has ever withstood our might. The Flood, too, shall fail
~ Truth in the mission High Charity
I opened the portal to this hallowed place, this shelter from Halo's fire, in the hopes that more of our Covenant would join us. Alas, save for a rabble of Heretics and their Demon allies (Humans), we are all that remains on this new world. So we must temper joy and sorrow in our hearts, for those who were left behind
~ Truth in The Ark
Your forefathers wisely set aside their compassion...steeled themselves for what needed to be done. I see now why they left you behind. You were weak... and gods must be strong!