Chasm King is a NPC.
The first time you meet the Chasm King is when he brings you into the Mirror World, in the Beginner's Village. He gives instructions on your first quest depending on your race. After completing the mission, he will teach you to trap Monsters by using a Level 10 Monster-Trapping Mirror, and gives you Revival Point Portal Penny before you leave.
Once you step out of the Beginner's Village, he will assist you for your first 10 levels.
Upon reaching level 20 on any job, he will briefly tell you about Life Quests.
If you decide to ignore the Chasm King when he appears, he will keep following you and will try to talk to you. If you still refuse to interact with him, he will leave you alone.