What happened on this land around c. 323 BCE (Hellenistic successor period begins)?

Gradual decline of Seleucid rule in Iranian lands; rise of Parthian (Arsacid) monarchy and a return to Iranian political dominance in the plateau.
Rulers
Seleucid Empire
c. 312–150s BCE
Diadochi successor state controlling large parts of former Achaemenid territories including much of Iran.
Local Iranian dynasts (Parni/Parthians)
c. 250–100 BCE
Parni/Arsacid rise in northeastern Iran leading to Parthian power.
Battles
Arsacid uprisings and conflicts vs. Seleucids
c. 238 BCE
Arsacid/Parthian successes in east reduce Seleucid control.