Antasma is an evil bat king and one of the two main antagonists in Mario & Luigi: Dream Team, along with Bowser. His name likely derives from "fantasma," the word for "ghost" in various Romance languages, and the prefix "anti-."
As told by a Master Pi'illo in west Somnom Woods, at one point, many years ago, Antasma was a normal bat living on Pi'illo Island. However, he once fed on the nightmares of a Pi'illo, and continued to feed on nightmares afterward, slowly turning him into his current self.
Setting his eyes on the Dark Stone, Antasma stole it for himself. The Pi'illos, along with the Zeekeeper, fought Antasma to stop his wishes from coming true and managed to imprison him into the Dream World; however, just before his seal was complete, Antasma shattered the Dark Stone, causing the chunks to rain down all over the island. The shards of the Dark Stone turned all the Pi'illo they touched into stone, causing the demise of the Pi'illo civilization and triggering most subsequent events in the game.
And he is voiced by Frank Welker in the English dub and he is reprised by Akiko Sugimoto in the Japanese dub.