They do not appear on the map by default, and must be awakened by the Warriors of Chaos. In order for them to be awakened, the Warriors of Chaos must win a battle against a settlement in the Helspire Mountains. It doesn't matter who currently controls the settlement.
If the Graelings are wiped out, the Warriors of Chaos can just awaken them again by winning another battle against a settlement in the Helspire Mountains.
Graeling is a tribe of Norscans that are frequently represented by champions of Khorne. One of Archaon the Everchosen's, lieutenants Haargroth the Blooded, was the jarl of this tribe.