Where did the Thracians go?

#Thracians #Spartacus #Thracia
Herodotus called the Thracians the most numerous people in the world, after the Indians.
Their warriors regularly fought as part of the Greek and Persian troops, and only Philip of Macedon was able to conquer their kingdom.
Nevertheless, the Thracians disappeared without a trace long before the era of the Great Migration of Nations!
Where could such a large ethnic group disappear to?