What is "chandrahas"

In the Hindu epic Ramayana, the Chandrahas sword is an indestructible sword that Lord Shiva gifts Ravana. It is believed that Ravana went travelling in his chariot, he came across Mount Kailash. He became furious as his chariot could not cross over this mountain, so in a state of fury he ordered the mountain to give way to him. The mountain refused as it was the residing place of Lord Shiva and Goddess Parvati. Ravana stepped down the chariot and lifted the mountain to remove it from his path. Lord Shiva placed his toe on the mountain to put it back into place. This caused Ravana's little finger to crush under the mountain. Ravana realised his mistake and to impress Lord Shiva sang the Shiva Tandava Stotram for the first time. Lord Shiva, impressed by his offering, gifted him the Chandrahas sword which is indestructible.