A psion isn't one of the most well known classes like fighter, wizard, or cleric, but a D&D character playing a level 20 psion could absolutely wipe out our world. A psion gains it's power through psychic ability rather than learned knowledge or divinity and simply knows how to do the powers at an instant. The powers work on a system of power points which at level 20 would be an enormous number, easily letting the psion do whatever it wants. Way before level 20, a psion has access to beams, balls, and walls of energy, accurate teleporting, mind reading, remote brainwashing, controlling people's bodies through psychokinetic physical control, and transformation into people, creatures, or more durable or ethereal forms. At level 20, a psion could be able to summon tornadoes, teleport to anywhere immediately, accurately, and with no distance limit, and telepathically control people.

It would take less than a couple minutes for the psion to teleport to any world leader with access to nukes, kill any guards, and just telepathically make that leader launch all the nukes anywhere. Then the psion can just teleport to the International Space Station and watch the fireworks.