Ragnar

Ragnar is the Obsidium Pre-Hardmode boss.

Spawn
Ragnar doesn't spawn on his own, and requires the player to summon him with a Molten Mess in the Obsidium.

Behavior
Ragnar will move above the player erratically and charges vertically when he's done firing rocks.

Attacks:
 * When he comes in contact with a tile or player he will let out a burst of rocks that inflict the On Fire! debuff for 2 seconds.
 * He will shoot a barrel rock at the player that can explode into multiple lava rocks.
 * At 25% health and below, he will shoot two barrel of rocks and shoots more multiple rocks.

In Expert Mode:
 * His damage and rocks have been increased.

Tips

 * Using Obsidian Skin Potion or Obsidian Skin Gem will help as you will be immune to the debuff On Fire!.
 * Building an arena in the Obsidium can be tricky as it covered terrarian with Obsidian; so blowing up the area or mining is required.