In the Temple of Veeshan: Hits for a max ~1,200; has ~310,000 hps. Every 12 seconds, "Scream of Chaos" is cast from an unknown source (a knockback in the direction Lord Vyemm is currently facing). He casts "Call of the Zero" every six seconds. (Picture is courtesy Afterlife.)
In the Corrupted Temple of Veeshan: See this quest entry for event information.