First Horizon Corp. and TD Bank Group have mutually agreed to end their merger agreement, which was valued at $13.4 billion when it was first announced more than a year ago, the companies announced Thursday morning. First Horizon shares were plummeting 45% in premarket trading after the announcement. Prior to the announcement, First Horizon stock had been trading well below the $25-per-share deal price, a sign that investors were skeptical that the deal would go through.