Telmar was a land inhabited by the Telmarine people, who were descendants of pirates originally from Earth. By 1998, their land had suffered a famine, which was also during the time of Narnia's Dark Age. The Narnians were in a state of anarchy without any remaining monarchs, due to the fact that their original monarchs had all mysteriously disappeared centuries earlier, leaving no heirs, resulting in their kingdom suffering social collapse and loss of prosperity.
Once Caspian I discovered this, he saw an opportunity to seize control of Narnia, thus gaining a new and fertile land for his kingdom. He mobilized the Telmarine army and attacked Narnia, defeating its army and killing many of the creatures who were native to the land.
Many creatures acted dumb or fled into the Shuddering Wood to avoid capture and death, and over time the Telmarine monarchs turned Narnia into a human kingdom. Some of them even attempted to destroy the past by letting forests grow on the coast, creating ideas of ghosts haunting the sea, and building a new capitol in the form of a castle, letting Cair Paravel become an island off the coast of the mainland by a strait.
Thus the Telmarine Age had begun.