In East Wastes, Zeixshi-Kar's Shard: Located at 230, 4900, 400 -- extreme northwestern corner of the zone (also findable with CTRL+F).
In East Wastes, Zeixshi-Kar's Shard (instanced): Dirgen appears in the group mission "Defeat Zeixshi-Kar" (see this quest entry for details) and the raid event "Zeixshi-Kar's Shard" (see this quest entry for details).
For all things Rain of Fear, see this overview of the expansion.