Excommunication was a rare and harsh punishment that stripped a paladin of his light-given powers, banished him from the paladinship and send him into exile. Though it had only been used a few times, every Paladin lived in mortal fear of it.
Tirion Fordring was excommunicated and exiled by Uther the Lightbringer after he had assaulted Alliance soldiers during the capture of the orc Eitrigg. When his Light-given powers have been stripped from his body with a sweeping motion of Uther's hand, Tirion felt panic and despair overwhelmed him as the grace and strengthening energies of the Light fled his body, he felt as if he had been wrapped in darkness and cast down into oblivion, and then observed the world darkening around him despite the ambient light not fading. He realized that he could no longer use his sacred powers, and was unable to withstand the raging despair and hopelessness that continued to wash over him in waves.
Tirion's story however shows later that even if the power of the Light has been stripped of his body and that he could no longer use it, his connection cannot be stripped by men, as he was able to re-establish it and heal an injured Eitrigg by his desperate will to save him, and used the Light even after that in the years and wars to come.