Commander Alast Degmar (click for map image) spawns in the throne room in the eastern wing of the castle (PH "Sub Commander Hob Stetson").
Map point for "degmar.txt" map files (for all Degmar named map points, see this quest entry):
He hits for a max ~36,000. Every 50 seconds, casts "Degmar's Wrath" on everyone on hate list (27,000 hp/tick DOT for 3 ticks; 38 corruption counters). Every 20 seconds, casts "Repentance" on a random single target (2,750-point reverse damage shield + 23,000/tick DOT for 3 ticks; 65 curse counters).
In the Heroic Adventure (Castle Relic): He doesn't hit quite as hard (max ~29,000) and doesn't cast any spells.