Last Prism

The Last Prism is a drop from the Moon Lord.

There appear to be 6 beams that deal damage that rotate and get closer together as the beam charges, this takes approx. 4 seconds, each beam does damage once per second. This means without crits or damage amplification this weapon does 600 damage per second when fully charged. However due to the high mana cost (12 mana 6 times per second when fully charged is 72 mana per second) players will struggle to maintain the beam for more than 5-6 seconds. A Mana Flower is recommended for this weapon. The beam does not pass through blocks but can pass through platforms.

Trivia
The life form disintegration rainbow may relate to the MOBA game League of Legends, where a champion, Vel' Koz has an ultimate skill called "Lifeform Disintegration Ray."

Or maybe from bullet hell game Touhou Project, where the character Kirisame Marisa (Japanese name format) is infamous for having stolen the spellcard Love Sign: Master Spark that looks like huge rainbow laser beam that deals massive damage.

Or even from paper and pen game Dungeons & Dragons, where monster named Beholder has similiar abilities.