"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!"
Religious fanatic and one of the hierarchs of the Covenant, the Prophet of Truth commands his forces with a calm but passionate demeanor. He is constantly spouting forth propaganda to boost the morale of the various species that make up the Covenant, while demonizing the humans they are in a constant race against to recover Forerunner artifacts.
Though the three Prophets of Truth, Mercy, and Regret all technically oversee the Covenant and help to maintain its path toward the Great Journey, the events of Halo 2 make it abundantly clear that Truth is the one pulling most of the strings. While the Covenant forces respect each Prophet, following their commands without question, Truth is the one Tartarus, the Arbiter, and Half-Jaw all turn to for their next orders. He is always at the forefront during any proclamations to the Covenant masses.
When Regret makes his preemptive strike on Earth, Truth informs him that his actions nearly jeopardized the Great Journey. When Master Chief crosses paths with Regret and kills him, Truth orders a ship to fire on their location, implying that he would have killed Truth whether Chief beat him to it or not - Master Chief's presence there was merely an excuse. Toward the end of Halo 2, when the Prophets and the Brutes are leaving High Charity for Earth, a flood spore attacks Mercy. Tartarus tries to help him, but Truth stops him, claiming that the Great Journey will wait for no one. With both Mercy and Regret out of the picture, Truth officially assumes complete control of the Covenant.
There are a few things that Truth indirectly caused without realizing the impact it would have. In ordering Tartarus and the Arbiter to retrieve the Icon on Delta Halo, his unleashed the Flood from containment, allowing the Gravemind to leave, claim High Charity, and invade both Earth and the Ark. Also, Truth's aims are misguided. He believes that activating the Halo array will lead to the Great Journey, some sort of spiritual transcendence, when in reality, activation of the rings will wipe out all life in the galaxy.
However, Truth knows full well that the humans want information from any and all Forerunner technology they can get their hands on, and sees this as a threat to the Great Journey. He sees humans as a significant threat to the Covenant, as well as the Separatist forces. In Halo 3, the Covenant Separatists are a full-fledged fighting force allied with the humans of the UNSC. But in Halo 2 they are scattered rebel groups, and Truth orders the Arbiter to quell them with edge of his blade.
Truth comes very close to achieving his dream, initiating the entire Halo array at once near the end of Halo 3. But Master Chief and the Arbiter manage to stop him just before the rings can fire. The Arbiter then kills the Prophet who tarnished his image.
Truth always keeps his wits about him, save perhaps in his final hours, when he sends his forces out in desperation, hoping they can buy him enough time to complete his work. For someone so delusional, Truth is always depicted as calm and confident, if not power-hungry. And the fact that he was able to command so many who would follow his word to their dying breaths says a lot about his leadership and manipulation skills.
There isn't ever any boss fight against Truth. But the Sacred Icon Suite from Halo 2's soundtrack is perhaps the single tune most closely tied to him. Plus, it's easily one of the best tunes from any of the Halo games.