Found in Blackwing Lair.
Each Pack contains one or two. Seethers are moderately tough level 62+ mobs with ~110k HP. They go into a frenzy state to inflict additional melee damage (can be removed with tranquilizing shot!), and have a frontal cleave attack. They cannot be slept or otherwise crowdcontrolled, and must be tanked instead.
As of patch 1.12 Seethers now have a Flame Buffet ability that deals roughly 1k damage per stack. Be aware of this if you kite these.